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Washington, DC, United States

The National Aeronautics and Space Administration is the United States government agency responsible for the civilian space program as well as aeronautics and aerospace research.President Dwight D. Eisenhower established the National Aeronautics and Space Administration in 1958 with a distinctly civilian orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29, 1958, disestablishing NASA's predecessor, the National Advisory Committee for Aeronautics . The new agency became operational on October 1, 1958.Since that time, most U.S. space exploration efforts have been led by NASA, including the Apollo moon-landing missions, the Skylab space station, and later the Space Shuttle. Currently, NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle, the Space Launch System and Commercial Crew vehicles. The agency is also responsible for the Launch Services Program which provides oversight of launch operations and countdown management for unmanned NASA launches.NASA science is focused on better understanding Earth through the Earth Observing System, advancing heliophysics through the efforts of the Science Mission Directorate's Heliophysics Research Program, exploring bodies throughout the Solar System with advanced robotic missions such as New Horizons, and researching astrophysics topics, such as the Big Bang, through the Great Observatories and associated programs. NASA shares data with various national and international organizations such as from the Greenhouse Gases Observing Satellite. Wikipedia.

Canuto V.M.,NASA | Canuto V.M.,Columbia University
Astronomy and Astrophysics | Year: 2011

In this paper we use the Reynolds stress models (RSM) to derive algebraic expressions for the following variables: a) heat fluxesμb) fluxes; and c) momentum fluxes. These relations, which are fully 3D, include: 1) stable and unstable stratification, represented by the Brunt-Väisäla frequency, N2 = -gH.-1p (∇- ∇ad)(1 . Rμ;); 2) double diffusion, salt-fingers, and semi-convection, represented by the density ratio Rμ; = .∇μ(∇- ∇ad)-1; 3) shear (differential rotation), represented by the mean squared shear σ2 or by the Richardson number, Ri = N2σ-2; 4) radiative losses represented by a Peclet number, Pe; 5) a complete analytical solution of the 1D version of the model. In general, the model requires the solution of two differential equations for the eddy kinetic energy K and its rate of dissipation, η. In the local and stationary cases, when production equals dissipation, the model equations are all algebraic. © 2011 ESO. Source

Kane S.R.,San Francisco State University | Gelino D.M.,NASA
Astrophysical Journal | Year: 2014

There are numerous multi-planet systems that have now been detected via a variety of techniques. These systems exhibit a range of both planetary properties and orbital configurations. For those systems without detected planetary transits, a significant unknown factor is the orbital inclination. This produces an uncertainty in the mass of the planets and their related properties, such as atmospheric scale height. Here we investigate the HD 10180 system, which was discovered using the radial velocity technique. We provide a new orbital solution for the system which allows for eccentric orbits for all planets. We show how the inclination of the system affects the mass/radius properties of the planets and how the detection of phase signatures may resolve the inclination ambiguity. We finally evaluate the Habitable Zone properties of the system and show that the g planet spends 100% of an eccentric orbit within the Habitable Zone. © 2014. The American Astronomical Society. All rights reserved.. Source

Bodenheimer P.,University of California at Santa Cruz | Lissauer J.J.,NASA
Astrophysical Journal | Year: 2014

Formation of planets in the Neptune size range with low-mass, but voluminous, H2/He gaseous envelopes is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. Formation locations ranging from 0.5 to 4 AU from a star of 1 M⊙ are considered. The final planets have heavy-element cores of 2.2-2.5 M ⊕ and envelopes in the range 0.037-0.16 M ⊕. After the formation process, which lasts 2 Myr or less, the planets evolve at constant mass up to an age of several Gyr. For assumed equilibrium temperatures of 250, 500, and 1000 K, their calculated final radii are compared with those observed by the Kepler spacecraft. For the particular case of Kepler-11 f, we address the question whether it could have formed in situ or whether migration from a formation location farther out in the disk is required. © 2014. The American Astronomical Society. All rights reserved.. Source

Gatlin P.N.,University of Alabama in Huntsville | Goodman S.J.,NASA
Journal of Atmospheric and Oceanic Technology | Year: 2010

An algorithm that provides an early indication of impending severe weather from observed trends in thunderstorm total lightning flash rates has been developed. The algorithm framework has been tested on 20 thunderstorms, including 1 nonsevere storm, which occurred over the course of six separate days during the spring months of 2002 and 2003. The identified surges in lightning rate (or jumps) are compared against 110 documented severe weather events produced by these thunderstorms as they moved across portions of northern Alabama and southern Tennessee. Lightning jumps precede 90% of these severe weather events, with as much as a 27-min advance notification of impending severe weather on the ground. However, 37% of lightning jumps are not followed by severe weather reports. Various configurations of the algorithm are tested, and the highest critical success index attained is 0.49. Results suggest that this lightning jump algorithm may be a useful operational diagnostic tool for severe thunderstorm potential. © 2010 American Meteorological Society. Source

Stuchlik D.W.,NASA
AIAA Balloon Systems Conference, 2015 | Year: 2015

The National Aeronautics and Space Administration’s (NASA) Balloon Program Office (BPO) is always looking for ways to provide new capabilities for science users and to improve their scientific return. The Wallops Arc Second Pointer (WASP) is the result of an in-house technology development effort at NASA’s Wallops Flight Facility (WFF) to produce a balloon borne fine pointing system which can point telescopes and other instruments on balloon gondolas at targets with arc-second accuracy and stability. WASP is a flexible system that can be used to support a variety of science-provided instruments with a wide range of masses and inertias to meet specific mission performance requirements. Major components of WASP are reusable which reduces the overall costs to the NASA BPO and to users. Arc-second pointing accuracy can be used to support a number of different science disciplines such as Planetary, Earth Science, and Astrophysics. The WASP system components will be described, and the development history, gondola configurations of the five WASP test flights, and pointing performance will be presented. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Source

We have constructed an analytic formula for the mean radial modulation transfer function of the best-corrected human eye as a function of pupil diameter, based on previously collected wave front aberrations from 200 eyes (Thibos, Hong, Bradley, & Cheng, 2002). This formula will be useful in modeling the early stages of human vision. Source

Kelly P.L.,University of California at Berkeley | Filippenko A.V.,University of California at Berkeley | Modjaz M.,New York University | Kocevski D.,NASA
Astrophysical Journal | Year: 2014

Spectra of broad-lined Type Ic supernovae (SNe Ic-BL), the only kind of SN observed at the locations of long-duration gamma-ray bursts (LGRBs), exhibit wide features indicative of high ejecta velocities (0.1c). We study the host galaxies of a sample of 245 low-redshift (z < 0.2) core-collapse SNe, including 17 SNe Ic-BL, discovered by galaxy-untargeted searches, and 15 optically luminous and dust-obscured z < 1.2 LGRBs. We show that, in comparison with Sloan Digital Sky Survey galaxies having similar stellar masses, the hosts of low-redshift SNe Ic-BL and z < 1.2 LGRBs have high stellar mass and star formation rate densities. Core-collapse SNe having typical ejecta velocities, in contrast, show no preference for such galaxies. Moreover, we find that the hosts of SNe Ic-BL, unlike those of SNe Ib/Ic and SNe II, exhibit high gas velocity dispersions for their stellar masses. The patterns likely reflect variations among star-forming environments and suggest that LGRBs can be used as probes of conditions in high-redshift galaxies. They may be caused by efficient formation of massive binary progenitor systems in densely star-forming regions, or, less probably, a higher fraction of stars created with the initial masses required for an SN Ic-BL or LGRB. Finally, we show that the preference of SNe Ic-BL and LGRBs for galaxies with high stellar mass and star formation rate densities cannot be attributed to a preference for low metal abundances but must reflect the influence of a separate environmental factor. © 2014. The American Astronomical Society. All rights reserved. Source

Stacy A.,NASA | Stacy A.,University of California at Berkeley | Bromm V.,University of Texas at Austin
Astrophysical Journal | Year: 2014

We perform numerical simulations of the growth of a Population III stellar system under photodissociating feedback. We start from cosmological initial conditions at z = 100, self-consistently following the formation of a minihalo at z = 15 and the subsequent collapse of its central gas to high densities. The simulations resolve scales as small as 1 AU, corresponding to gas densities of 1016 cm-3. Using sink particles to represent the growing protostars, we evolve the stellar system for the next 5000 yr. We find that this emerging stellar group accretes at an unusually low rate compared with minihalos which form at earlier times (z = 20-30), or with lower baryonic angular momentum. The stars in this unusual system will likely reach masses ranging from <1 M to 5 M by the end of their main-sequence lifetimes, placing them in the mass range for which stars will undergo an asymptotic giant branch (AGB) phase. Based upon the simulation, we predict the rare existence of Population III stars that have survived to the present day and have been enriched by mass overflow from a previous AGB companion. © 2014. The American Astronomical Society. All rights reserved.. Source

Parris M.G.,NASA | Sharma C.A.,University of Central Florida | Demara R.F.,University of Central Florida
ACM Computing Surveys | Year: 2011

The capabilities ofcurrent fault-handling techniques for Field Programmable Gate Arrays (FPGAs) develop a descriptive classification ranging from simple passive techniques to robust dynamic methods. Fault-handling methods not requiring modification of the FPGA device architecture or user intervention to recover from faults are examined and evaluated against overhead-based and sustainability-based performance metrics such as additional resource requirements, throughput reduction, fault capacity, and fault coverage. This classification alongside these performance metrics forms a standard for confident comparisons. © 2011 ACM. Source

Rozier K.Y.,NASA
Computer Science Review | Year: 2011

We are seeing an increased push in the use of formal verification techniques in safety-critical software and hardware in practice. Formal verification has been successfully used to verify systems such as air traffic control, airplane separation assurance, autopilot, CPU designs, life-support systems, medical equipment (such as devices which administer radiation), and many other systems which ensure human safety. This survey provides a perspective on the formal verification technique of linear temporal logic (LTL) symbolic model checking, from its history and evolution leading up to the state-of-the-art. We unify research from 1977 to 2009, providing a complete end-to-end analysis embracing a users' perspective by applying each step to a real-life aerospace example. We include an in-depth examination of the algorithms underlying the symbolic model-checking procedure, show proofs of important theorems, and point to directions of ongoing research. The primary focus is on model checking using LTL specifications, though other approaches are briefly discussed and compared to using LTL. © 2010. Source

Patsourakos S.,University of Ioannina | Klimchuk J.A.,NASA | Young P.R.,George Mason University
Astrophysical Journal | Year: 2014

Recent solar spectroscopic observations have shown that coronal spectral lines can exhibit asymmetric profiles, with enhanced emissions at their blue wings. These asymmetries correspond to rapidly upflowing plasmas at speeds exceeding κ50 km s-1. Here, we perform a study of the density of the rapidly upflowing material and compare it with that of the line core that corresponds to the bulk of the plasma. For this task, we use spectroscopic observations of several active regions taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode mission. The density sensitive ratio of the Fe XIV lines at 264.78 and 274.20 Å is used to determine wing and core densities. We compute the ratio of the blue wing density to the core density and find that most values are of order unity. This is consistent with the predictions for coronal nanoflares if most of the observed coronal mass is supplied by chromospheric evaporation driven by the nanoflares. However, much larger blue wing-to-core density ratios are predicted if most of the coronal mass is supplied by heated material ejected with type II spicules. Our measurements do not rule out a spicule origin for the blue wing emission, but they argue against spicules being a primary source of the hot plasma in the corona. We note that only about 40% of the pixels where line blends could be safely ignored have blue wing asymmetries in both Fe XIV lines. Anticipated sub-arcsecond spatial resolution spectroscopic observations in future missions could shed more light on the origin of blue, red, and mixed asymmetries. © 2014. The American Astronomical Society. All rights reserved. Source

Sahraoui F.,French National Center for Scientific Research | Goldstein M.L.,NASA | Belmont G.,Ecole Polytechnique - Palaiseau | Canu P.,Ecole Polytechnique - Palaiseau | Rezeau L.,Ecole Polytechnique - Palaiseau
Physical Review Letters | Year: 2010

We show the first three dimensional (3D) dispersion relations and k spectra of magnetic turbulence in the solar wind at subproton scales. We used the Cluster data with short separations and applied the k-filtering technique to the frequency range where the transition to subproton scales occurs. We show that the cascade is carried by highly oblique kinetic Alfvén waves with ωplas≤0.1ωci down to kρi∼2. Each k spectrum in the direction perpendicular to B0 shows two scaling ranges separated by a breakpoint (in the interval [0.4,1]kρi): a Kolmogorov scaling kρ-1.7 followed by a steeper scaling ∼kρ-4.5. We conjecture that the turbulence undergoes a transition range, where part of the energy is dissipated into proton heating via Landau damping and the remaining energy cascades down to electron scales where electron Landau damping may predominate. © 2010 The American Physical Society. Source

Hulot G.,CNRS Paris Institute of Global Physics | Sabaka T.J.,NASA
Space Science Reviews | Year: 2010

Observations of the Earth's magnetic field from low-Earth orbiting (LEO) satellites started very early on, more than 50 years ago. Continuous such observations, relying on more advanced technology and mission concepts, have however only been available since 1999. The unprecedented time-space coverage of this recent data set opened revolutionary new possibilities for monitoring, understanding and exploring the Earth's magnetic field. In the near future, the three-satellite Swarm constellation concept to be launched by ESA, will not only ensure continuity of such measurements, but also provide enhanced possibilities to improve on our ability to characterize and understand the many sources that produce this field. In the present paper we review and discuss the advantages and drawbacks of the various LEO space magnetometry concepts that have been used so far, and report on the motivations that led to the latest Swarm constellation concept. We conclude with some considerations about future concepts that could possibly be implemented to ensure the much needed continuity of LEO space magnetometry, possibly with enhanced scientific return, by the time the Swarm mission ends. © 2010 Springer Science+Business Media B.V. Source

Morelli E.A.,NASA
Journal of Aircraft | Year: 2012

Novel flight test maneuvers for efficient aerodynamic modeling were developed and demonstrated in flight. Orthogonal optimized multisine inputs were applied to aircraft control surfaces to excite aircraft dynamic response in all six degrees of freedom simultaneously, while keeping the aircraft close to chosen reference flight conditions. Each maneuver was designed for a specific modeling task that cannot be adequately or efficiently accomplished using conventional flight test maneuvers. The maneuvers are described and explained, then demonstrated on a subscale jet transport aircraft in flight. Real-time and post-flight modeling results from equation-error parameter estimation in the frequency domain were used to show the effectiveness and efficiency of the maneuvers, as well as the quality of the aerodynamic models that can be identified from the resultant flight data. Source

Nemmen R.S.,NASA | Storchi-Bergmann T.,Federal University of Rio Grande do Sul | Eracleous M.,Pennsylvania State University
Monthly Notices of the Royal Astronomical Society | Year: 2014

We perform an exploratory study of the physical properties of accretion flows and jets in low-luminosity active galactic nuclei (LLAGNs) by modelling the spectral energy distributions (SEDs) of 12 LLAGNs in low-ionization nuclear emission-line regions (LINERs). These SEDs we constructed from high-resolution radio, X-ray and optical/ultraviolet (UV) observations of the immediate vicinity of the black hole. We adopt a coupled accretion-jet model comprising an inner advection-dominated accretion flow (ADAF) and an outer standard thin disc. We present best-fitting models in which either the ADAF or the jet dominates the X-ray emission.Six sources in our sample display an optical-UV excess with respect to ADAF and jet models; this excess can be explained as emission from the truncated disc with transition radii 30-225 RS in four of them. In almost all sources the optical emission can also be attributed to unresolved, old stellar clusters with masses ̃107-108 M⊙. We find evidence for a correlation between the accretion rate and jet power and an anticorrelation between the radio loudness and the accretion rate. We confirm previous findings that the radio emission is severely underpredicted by ADAF models and explained by the relativistic jet. We find evidence for a non-linear relation between the X-ray and bolometric luminosities and a slight IR excess in the average model SED compared to that of quasars. We suggest that the hardness of the X-ray spectrum can be used to identify the X-ray emission mechanism and discuss directions for progress in understanding the origin of the X-rays. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Zhang K.,University of Montana | Kimball J.S.,University of Montana | Nemani R.R.,NASA | Running S.W.,University of Montana
Water Resources Research | Year: 2010

We applied a satellite remote sensing-based evapotranspiration (ET) algorithm to assess global terrestrial ET from 1983 to 2006. The algorithm quantifies canopy transpiration and soil evaporation using a modified Penman-Monteith approach with biome-specific canopy conductance determined from the normalized difference vegetation index (NDVI) and quantifies open water evaporation using a Priestley-Taylor approach. These algorithms were applied globally using advanced very high resolution radiometer (AVHRR) GIMMS NDVI, NCEP/NCAR Reanalysis (NNR) daily surface meteorology, and NASA/GEWEX Surface Radiation Budget Release-3.0 solar radiation inputs. We used observations from 34 FLUXNET tower sites to parameterize an NDVI-based canopy conductance model and then validated the global ET algorithm using measurements from 48 additional, independent flux towers. Two sets of monthly ET estimates at the tower level, driven by in situ meteorological measurements and meteorology interpolated from coarse resolution NNR meteorology reanalysis, agree favorably (root mean square error (RMSE) = 13.0-15.3 mm month-1; R2 = 0.80-0.84) with observed tower fluxes from globally representative land cover types. The global ET results capture observed spatial and temporal variations at the global scale and also compare favorably (RMSE = 186.3 mm yr-1; R2 = 0.80) with ET inferred from basin-scale water balance calculations for 261 basins covering 61% of the global vegetated area. The results of this study provide a relatively long term global ET record with well-quantified accuracy for assessing ET climatologies, terrestrial water, and energy budgets and long-term water cycle changes. Copyright 2010 by the American Geophysical Union. Source

Alperin M.,University of North Carolina at Chapel Hill | Hoehler T.,NASA
Science | Year: 2010

Our understanding of a major methane sink is based on studies of quiescent sediments and dynamic seeps. But do the same processes operate in both environments? Source

Zaman K.B.M.Q.,NASA
AIAA Journal | Year: 2012

Significant differences in subsonic jet noise databases have been reported in recent review papers. Specifically, university-type facilities involving higher contraction ratios and possibly cleaner flows are noted to yield higher levels of noise relative to data from industrial-type facilities. An experimental investigation is carried out in an attempt to understand the sources of the anomaly. It is inferred that differences in jet core turbulence may not be the source. An observation in a previous study is confirmed showing that two nozzles of the same diameter but different internal geometry can produce a difference in subsonic jet noise. The present measurements demonstrate that the noisier nozzle involves a highly disturbed laminar, or nominally laminar, boundary-layer state as opposed to a turbulent state with the other. The former boundary-layer state with the noisier nozzle is actually marked by larger turbulence intensities, consistent with the higher radiated noise. Although the boundary-layer characteristics were not reported with the earlier databases, the present results suggest that differences therein might be a source of the anomaly. Source

Castner R.,NASA
Journal of Aircraft | Year: 2012

Reducing or eliminating the operational restrictions of supersonic aircraft over populated areas has led to research on the sonic boom. To make the sonic boom acceptable, both the leading and trailing shocks need to be reduced. Progress has been made previously to reduce the leading shock through aircraft shaping. Analysis and testing for an isolated nozzle configuration was performed to study the trailing shock waves caused by the exhaust nozzle plume. Both computational fluid dynamics analysis and wind-tunnel testing show how the shock wave formed at the nozzle lip interacts with the nozzle boat-tail expansion wave. Results demonstrate how underexpanded nozzle flow can be associated with a reduction in the strength of the trailing shock wave. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. Source

Strohmayer T.,NASA | Mahmoodifar S.,University of Maryland University College
Astrophysical Journal | Year: 2014

We present results of targeted searches for signatures of non-radial oscillation modes (such as r- and g-modes) in neutron stars using RXTE data from several accreting millisecond X-ray pulsars (AMXPs). We search for potentially coherent signals in the neutron star rest frame by first removing the phase delays associated with the star's binary motion and computing fast Fourier transform power spectra of continuous light curves with up to 230 time bins. We search a range of frequencies in which both r- and g-modes are theoretically expected to reside. Using data from the discovery outburst of the 435 Hz pulsar XTE J1751-305 we find a single candidate, coherent oscillation with a frequency of 0.5727597 × νspin = 249.332609 Hz, and a fractional Fourier amplitude of 7.46 × 10-4. We estimate the significance of this feature at the 1.6 × 10-3 level, slightly better than a 3σ detection. Based on the observed frequency we argue that possible mode identifications include rotationally modified g-modes associated with either a helium-rich surface layer or a density discontinuity due to electron captures on hydrogen in the accreted ocean. In the latter case the presence of sufficient hydrogen in this ultracompact system with a likely helium-rich donor would present an interesting puzzle. Alternatively, the frequency could be identified with that of an inertial mode or a core r-mode modified by the presence of a solid crust; however, the r-mode amplitude required to account for the observed modulation amplitude would induce a large spin-down rate inconsistent with the observed pulse timing measurements. For the AMXPs XTE J1814-338 and NGC 6440 X-2 we do not find any candidate oscillation signals, and we place upper limits on the fractional Fourier amplitude of any coherent oscillations in our frequency search range of 7.8 × 10 -4 and 5.6 × 10-3, respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions. © 2014. The American Astronomical Society. All rights reserved. Source

Burlaga L.F.,NASA | Ness N.F.,Catholic University of America
Astrophysical Journal | Year: 2014

Voyager 1 (V1) has been observing interstellar magnetic fields for more than one year beginning 2012/209, when V1 crossed a current sheet, a "CS0" having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an interstellar magnetic field B draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed at 2012/167 and 2011/276 with high inclinations (99° ± 10° and 89° ± 10°, respectively). From 2013.0 to 2013.6, the difference between the azimuthal angle λ of B from the Parker spiral angle at the latitude 34.°6 of V1 was λ-λP = 22° ± 3° and the corresponding difference of the elevation angle δ was δ-δP = 23° ± 8°. During 2012, the deviation from the Parker spiral angle was somewhat smaller. The interstellar magnetic field has a "west to east polarity," opposite to the direction of planetary motions. The magnitude of B varied smoothly in the range 0.38-0.59 nT with an average B = 0.486 ± 0.045 after 2012/237.7. The transition from heliosheath to interstellar magnetic fields is related to a "two-step" increase in the cosmic ray intensity observed by V1 from 2012.30 to 2012.65. The first step increase began near the end of an unusual "away-polarity" sector, and it reached a plateau when V1 moved into a "toward-polarity" sector that ended at CS0. The second step increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728. © 2014. The American Astronomical Society. All rights reserved.. Source

Klimchuk J.A.,NASA | Bradshaw S.J.,Rice University
Astrophysical Journal | Year: 2014

It has been suggested that the hot plasma of the solar corona comes primarily from impulsive heating events, or nanoflares, that occur in the lower atmosphere, either in the upper part of the ordinary chromosphere or at the tips of type II spicules. We test this idea with a series of hydrodynamic simulations. We find that synthetic Fe XII (195) and Fe XIV (274) line profiles generated from the simulations disagree dramatically with actual observations. The integrated line intensities are much too faint; the blueshifts are much too fast; the blue-red asymmetries are much too large; and the emission is confined to low altitudes. We conclude that chromospheric nanoflares are not a primary source of hot coronal plasma. Such events may play an important role in producing the chromosphere and powering its intense radiation, but they do not, in general, raise the temperature of the plasma to coronal values. Those cases where coronal temperatures are reached must be relatively uncommon. The observed profiles of Fe XII and Fe XIV come primarily from plasma that is heated in the corona itself, either by coronal nanoflares or a quasi-steady coronal heating process. Chromospheric nanoflares might play a role in generating waves that provide this coronal heating. © 2014. The American Astronomical Society. All rights reserved.. Source

Gopalswamy N.,NASA | Yashiro S.,Catholic University of America
Astrophysical Journal Letters | Year: 2011

We determine the coronal magnetic field strength in the heliocentric distance range 6-23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfvén Mach number, and hence the Alfvén speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from ∼48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances. © 2011. The American Astronomical Society. All rights reserved. Source

Bradshaw S.J.,Rice University | Klimchuk J.A.,NASA
Astrophysical Journal, Supplement Series | Year: 2011

The "smoking gun" of small-scale, impulsive events heating the solar corona is expected to be the presence of hot (>5MK) plasma. Evidence for this has been scarce, but has gradually begun to accumulate due to recent studies designed to constrain the high-temperature part of the emission measure distribution. However, the detected hot component is often weaker than models predict and this is due in part to the common modeling assumption that the ionization balance remains in equilibrium. The launch of the latest generation of space-based observing instrumentation on board Hinode and the Solar Dynamics Observatory (SDO) has brought the matter of the ionization state of the plasma firmly to the forefront. It is timely to consider exactly what emission current instruments would detect when observing a corona heated impulsively on small scales by nanoflares. Only after we understand the full effects of nonequilibrium ionization can we draw meaningful conclusions about the plasma that is (or is not) present. We have therefore performed a series of hydrodynamic simulations for a variety of different nanoflare properties and initial conditions. Our study has led to several key conclusions. (1) Deviations from equilibrium are greatest for short-duration nanoflares at low initial coronal densities. (2) Hot emission lines are the most affected and are suppressed sometimes to the point of being invisible. (3) For the many scenarios we have considered, the emission detected in several of the SDO-AIA channels (131, 193, and 211 ) would be dominated by warm, overdense, cooling plasma. (4) It is difficult not to create coronal loops that emit strongly at 1.5MK and in the range 2-6MK, which are the most commonly observed kind, for a broad range of nanoflare scenarios. (5) The Fe XV (284.16 ) emission in most of our models is about 10 times brighter than the Ca XVII (192.82 ) emission, consistent with observations. Our overarching conclusion is that small-scale, impulsive heating inducing a nonequilibrium ionization state leads to predictions for observable quantities that are entirely consistent with what is actually observed. © 2011. The American Astronomical Society. All rights reserved.. Source

Hathaway D.H.,NASA
Solar Physics | Year: 2011

The latitudinal location of the sunspot zones in each hemisphere is determined by calculating the centroid position of sunspot areas for each solar rotation from May 1874 to June 2011. When these centroid positions are plotted and analyzed as functions of time from each sunspot cycle maximum, there appear to be systematic differences in the positions and equatorward drift rates as a function of sunspot cycle amplitude. If, instead, these centroid positions are plotted and analyzed as functions of time from each sunspot cycle minimum, then most of the differences in the positions and equatorward drift rates disappear. The differences that remain disappear entirely if curve fitting is used to determine the starting times (which vary by as much as eight months from the times of minima). The sunspot zone latitudes and equatorward drift measured relative to this starting time follow a standard path for all cycles with no dependence upon cycle strength or hemispheric dominance. Although Cycle 23 was peculiar in its length and the strength of the polar fields it produced, it too shows no significant variation from this standard. This standard law, and the lack of variation with sunspot cycle characteristics, is consistent with dynamo wave mechanisms but not consistent with current flux transport dynamo models for the equatorward drift of the sunspot zones. © 2011 Springer Science+Business Media B.V. Source

Uritsky V.M.,Catholic University of America | Davila J.M.,NASA
Astrophysical Journal | Year: 2015

Using data from the STEREO and SOHO spacecraft, we show that temporal organization of energy release events in the quiet solar corona is close to random, in contrast to the clustered behavior of flaring times in solar active regions. The locations of the quiet-Sun events follow the meso- and supergranulation pattern of the underling photosphere. Together with earlier reports of the scale-free event size statistics, our findings suggest that quiet solar regions responsible for bulk coronal heating operate in a driven self-organized critical state, possibly involving long-range Alfvénic interactions. © 2014. The American Astronomical Society. All rights reserved. Source

Young A.P.,University of California at Santa Cruz | Knysh S.,Eloret Corporation | Smelyanskiy V.N.,NASA
Physical Review Letters | Year: 2010

We simulate the quantum adiabatic algorithm (QAA) for the exact cover problem for sizes up to N=256 using quantum Monte Carlo simulations incorporating parallel tempering. At large N, we find that some instances have a discontinuous (first-order) quantum phase transition during the evolution of the QAA. This fraction increases with increasing N and may tend to 1 for N→. © 2010 The American Physical Society. Source

Van Dijk T.,University of Amsterdam | Fischer D.G.,NASA | Visser T.D.,Technical University of Delft | Wolf E.,University of Rochester
Physical Review Letters | Year: 2010

In the analysis of light scattering on a sphere it is implicitly assumed that the incident field is spatially fully coherent. However, under usual circumstances the field is partially coherent. We generalize the partial waves expansion method to this situation and examine the influence of the degree of coherence of the incident field on the radiant intensity of the scattered field in the far zone. We show that when the coherence length of the incident field is comparable to, or is smaller than, the radius of the sphere, the angular distribution of the radiant intensity depends strongly on the degree of coherence. The results have implications, for example, for scattering in the atmosphere and colloidal suspensions. © 2010 The American Physical Society. Source

Remsberg E.E.,NASA
Atmospheric Chemistry and Physics | Year: 2015

This study makes use of time series of methane (CH4) data from the Halogen Occultation Experiment (HALOE) to detect whether there were any statistically significant changes of the Brewer-Dobson circulation (BDC) within the stratosphere during 1992-2005. The HALOE CH4 profiles are in terms of mixing ratio versus pressure altitude and are binned into latitude zones within the Southern Hemisphere and the Northern Hemisphere. Their separate time series are then analyzed using multiple linear regression (MLR) techniques. The CH4 trend terms for the Northern Hemisphere are significant and positive at 10° N from 50 to 7 hPa and larger than the tropospheric CH4 trends of about 3% decadeg'1 from 20 to 7 hPa. At 60° N the trends are clearly negative from 20 to 7 hPa. Their combined trends indicate an acceleration of the BDC in the middle stratosphere of the Northern Hemisphere during those years, most likely due to changes from the effects of wave activity. No similar significant BDC acceleration is found for the Southern Hemisphere. Trends from HALOE H2O are analyzed for consistency. Their mutual trends with CH4 are anti-correlated qualitatively in the middle and upper stratosphere, where CH4 is chemically oxidized to H2O. Conversely, their mutual trends in the lower stratosphere are dominated by their trends upon entry to the tropical stratosphere. Time series residuals for CH4 in the lower mesosphere also exhibit structures that are anti-correlated in some instances with those of the tracer-like species HCl. Their occasional aperiodic structures indicate the effects of transport following episodic, wintertime wave activity. It is concluded that observed multi-year, zonally averaged distributions of CH4 can be used to diagnose major instances of wave-induced transport in the middle atmosphere and to detect changes in the stratospheric BDC. © Author(s) 2015. CC Attribution 3.0 License. Source

Kahn R.A.,NASA | Gaitley B.J.,Jet Propulsion Laboratory
Journal of Geophysical Research D: Atmospheres | Year: 2015

In addition to aerosol optical depth (AOD), aerosol type is required globally for climate forcing calculations, constraining aerosol transport models and other applications. However, validating satellite aerosol-type retrievals is more challenging than testing AOD results, because aerosol type is a more complex quantity, and ground truth data are far less numerous and generally not as robust. We evaluate the Multiangle Imaging Spectroradiometer (MISR) Version 22 aerosol-type retrievals by assessing product self-consistency on a regional basis and by making comparisons with general expectation and with the Aerosol Robotic Network aerosol-type climatology, as available. The results confirm and add detail to the observation that aerosol-type discrimination improves dramatically where midvisible AOD exceeds about 0.15 or 0.2. When the aerosol-type information content of the observations is relatively low, increased scattering-angle range improves particle-type sensitivity. The MISR standard, operational product discriminates among small, medium, and large particles and exhibits qualitative sensitivity to single-scattering albedo (SSA) under good aerosol-type retrieval conditions, providing a categorical aerosol-type classification. MISR Ångström exponent deviates systematically from ground truth where particle types missing from the algorithm climatology are present, or where cloud contamination is likely to occur, and SSA tends to be overestimated where absorbing particles are found. We determined that the number of mixtures passing the algorithm acceptance criteria (#SuccMix) represents aerosol-type retrieval quality effectively, providing a useful aerosol-type quality flag. Key Points MISR multiangle retrievals provide global aerosol-type classification mapping Aerosol-type discrimination improves dramatically for aerosol optical depth 0.2 The number of successful mixtures represents aerosol-type retrieval quality ©2015. This article is a U.S. Government work and is in the public domain in the USA. Source

Understanding the origin of Martian methane will require numerous complementary measurements from both in situ and remote sensing investigations and laboratory work to correlate planetary surface geophysics with atmospheric dynamics and chemistry. Three instruments (Quadrupole Mass Spectrometer (QMS), Gas Chromatograph (GC) and Tunable Laser Spectrometer (TLS)) with sophisticated sample handling and processing capability make up the Sample Analysis at Mars (SAM) analytical chemistry suite on NASAs 2011 Mars Science Laboratory (MSL) Mission. Leveraging off the SAM sample and gas processing capability that includes methane enrichment, TLS has unprecedented sensitivity for measuring absolute methane (parts-per-trillion), water, and carbon dioxide abundances in both the Martian atmosphere and evolved from heated soil samples. In concert with a wide variety of associated trace gases (e.g. SO2, H 2S, NH3, higher hydrocarbons, organics, etc.) and other isotope ratios measured by SAM, TLS will focus on determining the absolute abundances of methane, water and carbon dioxide, and their isotope ratios: 13C/12C and D/H in methane; 13C/12C and 18O/17O/16O in carbon dioxide; and 18O/17O/16O and D/H in water. Measurements near the MSL landing site will be correlated with satellite (Mars Express, Mars 2016) and ground-based observations. © 2010 Elsevier Ltd. Source

Anderson C.M.,NASA | Samuelson R.E.,University of Maryland University College
Icarus | Year: 2011

Vertical distributions and spectral characteristics of Titan's photochemical aerosol and stratospheric ices are determined between 20 and 560cm-1 (500-18μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined. In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ∼3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at ∼160cm-1, appear to be located over a narrow altitude range in the stratosphere centered at ∼90km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S. There is some evidence of a second ice cloud layer at ∼60km altitude at 58°S associated with an emission feature at ∼80cm-1. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50-100km) associated with organic condensate clouds, Titan's photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3-2.4 at about 160km to 1.1-1.4 at 300km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ∼140cm-1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics. Finally, volume extinction coefficients NχE derived from 15°S CIRS data at a wavelength of λ=62.5μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583μm. This comparison yields volume extinction coefficient ratios NχE(1.583μm)/NχE(62.5μm) of roughly 70 and 20, respectively, for Titan's aerosol and stratospheric ices. The inferred particle cross-section ratios χE(1.583μm)/χE(62.5μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles. © 2011. Source

Emery J.P.,University of Tennessee at Knoxville | Burr D.M.,University of Tennessee at Knoxville | Cruikshank D.P.,NASA
Astronomical Journal | Year: 2011

The Trojan asteroids, a very substantial population of primitive bodies trapped in Jupiter's stable Lagrange regions, remain quite poorly understood. Because they occupy these orbits, the physical properties of Trojans provide a unique perspective on the chemical and dynamical processes that shaped the Solar System. The current study was therefore undertaken to investigate surface compositions of these objects. We present 66 new near-infrared (NIR; 0.7-2.5 μm) spectra of 58 Trojan asteroids, including members of both the leading and trailing swarms. We also include in the analysis previously published NIR spectra of 13 Trojans (3 of which overlap with the new sample). This data set permits not only a direct search for compositional signatures, but also a search for patterns that may reveal clues to the origin of the Trojans. We do not report any confirmed absorption features in the new spectra. Analysis of the spectral slopes, however, reveals an interesting bimodality among the NIR data. The two spectral groups identified appear to be equally abundant in the leading and trailing swarms. The spectral groups are not a result of family membership; they occur in the background, non-family population. The average albedos of the two groups are the same within uncertainties (0.051 ±0.016 and 0.055 ±0.016). No correlations between spectral slope and any other physical or orbital parameter are detected, with the exception of a possible weak correlation with inclination among the less-red spectral group. The NIR spectral groups are consistent with a similar bimodality previously suggested among visible colors and spectra. Synthesizing the present results with previously published properties of Trojans, we conclude that the two spectral groups represent objects with different intrinsic compositions. We further suggest that whereas the less-red group originated near Jupiter or in the main asteroid belt, the redder spectral group originated farther out in the Solar System. If this suggestion is correct, the Trojan swarms offer the most readily accessible large reservoir of Kuiper Belt material as well as a unique reservoir for the study of material from the middle part of the solar nebula. © 2011 The American Astronomical Society. All rights reserved. Source

Moore J.M.,NASA | Pappalardo R.T.,Jet Propulsion Laboratory
Icarus | Year: 2011

All landforms on Titan that are unambiguously identifiable can be explained by exogenic processes (aeolian, fluvial, impact cratering, and mass wasting). Previous suggestions of endogenically produced cryovolcanic constructs and flows have, without exception, lacked conclusive diagnostic evidence. The modification of sparse recognizable impact craters (themselves exogenic) can be explained by aeolian and fluvial erosion. Tectonic activity could be driven by global thermal evolution or external forcing, rather than by active interior processes. A lack of cryovolcanism would be consistent with geophysical inferences of a relatively quiescent interior: incomplete differentiation, only minor tidal heating, and possibly a lack of internal convection today. Titan might be most akin to Callisto with weather: an endogenically relatively inactive world with a cool interior. We do not aim to disprove the existence of any and all endogenic activity at Titan, nor to provide definitive alternative hypotheses for all landforms, but instead to inject a necessary level of caution into the discussion. The hypothesis of Titan as a predominantly exogenic world can be tested through additional Cassini observations and analyses of putative cryovolcanic features, geophysical and thermal modeling of Titan's interior evolution, modeling of icy satellite landscape evolution that is shaped by exogenic processes alone, and consideration of possible means for supplying Titan's atmospheric constituents that do not rely on cryovolcanism. © 2011. Source

McWilliams S.T.,NASA
Physical Review Letters | Year: 2010

Some braneworld models may have observable consequences that, if detected, would validate a requisite element of string theory. In the infinite Randall-Sundrum model (RS2), the AdS radius of curvature, ≤, of the extra dimension supports a single bound state of the massless graviton on the brane, thereby reproducing Newtonian gravity in the weak-field limit. However, using the AdS/CFT correspondence, it has been suggested that one possible consequence of RS2 is an enormous increase in Hawking radiation emitted by black holes. We utilize this possibility to derive two novel methods for constraining ≤ via gravitational wave measurements. We show that the EMRI event rate detected by LISA can constrain ≤ at the ∼1μm level for optimal cases, while the observation of a single galactic black hole binary with LISA results in an optimal constraint of ≤5μm. © 2010 The American Physical Society. Source

It is expected that the European Space Agency mission Gaia will make it possible to determine coordinates in the optical domain of more than 500,000 quasars. In 2006, a radio astrometry project was launched with the overall goal of making comparisons between coordinate systems derived from future space-born astrometry instruments and the coordinate system constructed from analysis of global very long baseline interferometry (VLBI) more robust. Investigation of the rotation, zonal errors, and non-alignment of the radio and optical positions caused by both radio and optical structures is needed to validate both techniques. In order to support these studies, the densification of the list of compact extragalactic objects that are bright in both radio and optical ranges is desirable. A set of 105 objects from the list of 398 compact extragalactic radio sources with decl. >-10°was observed with the Very Long Baseline Array and European VLBI Network (EVN) with the primary goal of producing images with milliarcsecond resolution. These sources are brighter than 18 mag in the V band, and they were previously detected by the EVN. In this paper, coordinates of observed sources have been derived with milliarcsecond accuracies from analysis of these VLBI observations using an absolute astrometry method. The catalog of positions for 105 target sources is presented. The accuracies of source coordinates are in the range of 0.3-7mas, with a median of 1.1mas. © 2011. The American Astronomical Society. All rights reserved. Source

Kane S.R.,NASA
Icarus | Year: 2011

With more than 15. years since the first radial velocity discovery of a planet orbiting a Sun-like star, the time baseline for radial velocity surveys is now extending out beyond the orbit of Jupiter analogs. The sensitivity to exoplanet orbital periods beyond that of Saturn orbital radii however is still beyond our reach such that very few clues regarding the prevalence of ice giants orbiting solar analogs are available to us. Here we simulate the radial velocity, transit, and photometric phase amplitude signatures of the Solar System giant planets, in particular Uranus and Neptune, and assess their detectability. We scale these results for application to monitoring low-mass stars and compare the relative detection prospects with other potential methods, such as astrometry and imaging. These results quantitatively show how many of the existing techniques are suitable for the detection of ice giants beyond the snow line for late-type stars and the challenges that lie ahead for the detection true Uranus/Neptune analogs around solar-type stars. © 2011 Elsevier Inc. Source

A highly accurate and computationally attractive shear-deformation theory for homogeneous, laminated composite, and sandwich laminates is developed for the linearly elastic analysis of planar beams. The theory is derived using the kinematic assumptions of Refined Zigzag Theory (RZT) and a two-step procedure that implements Reissner’s Mixed Variational Theorem (RMVT). The basic expression for the transverse-shear stress that satisfies a priori the equlibrium conditions along the layer interfaces is obtained from Cauchy’s equilibrium equations. The resulting transverse-shear stress consists of second-order derivatives of the two rotation variables of the theory, which subsequently are restated as the unknown stress functions. As the first step in fulfilling RMVT, the Lagrange-multiplier functional is minimized with respect to the unknown stress functions, resulting in the stress functions consisting of first-order derivatives of the kinematic variables. Subsequently, the second term of RMVT is minimized, producing four beam equilibrium equations and consistent boundary conditions. For any number of material layers the new theory maintains only four kinematic variables. The theory is labeled RZT(m), where the superscript (m) stands for mixed formulation. The RZT(m) can accurately model the axial stretch, bending, and transverse-shear deformations, without shear-correction factors. Analytic solutions are derived for simply supported beams subjected to transverse-normal and transverse-shear tractions on the top and bottom surfaces. It is demonstrated that RZT(m) has a wide range of applicability which includes sandwich construction and the laminates with embedded thin compliant layers that can potentially model progression of delaminations. The main advantage of RZT(m) over RZT is in the superior predictions of transverse-shear stresses that are obtained directly from the low-order transverse-shear strain measures of the theory without resorting to a post-processing integration procedure. Importantly, the methodology can be readily extended to plate theory, and it can be applied effectively for developing simple and efficient C0-continuous finite elements. © 2015 Springer Science+Business Media Dordrecht (outside the USA) Source

Wang T.,Catholic University of America | Wang T.,NASA
Space Science Reviews | Year: 2011

Strongly damped Doppler shift oscillations are observed frequently associated with flarelike events in hot coronal loops. In this paper, a review of the observed properties and the theoretical modeling is presented. Statistical measurements of physical parameters (period, decay time, and amplitude) have been obtained based on a large number of events observed by SOHO/SUMER and Yohkoh/BCS. Several pieces of evidence are found to support their interpretation in terms of the fundamental standing longitudinal slow mode. The high excitation rate of these oscillations in small- or micro-flares suggest that the slow mode waves are a natural response of the coronal plasma to impulsive heating in closed magnetic structure. The strong damping and the rapid excitation of the observed waves are two major aspects of the waves that are poorly understood, and are the main subject of theoretical modelling. The slow waves are found mainly damped by thermal conduction and viscosity in hot coronal loops. The mode coupling seems to play an important role in rapid excitation of the standing slow mode. Several seismology applications such as determination of the magnetic field, temperature, and density in coronal loops are demonstrated. Further, some open issues are discussed. © 2011 Springer Science+Business Media B.V. Source

Morris P.,NASA
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2014

An important issue for temporal planners is the ability to handle temporal uncertainty. Recent papers have addressed the question of how to tell whether a temporal network is Dynamically Controllable, i.e., whether the temporal requirements are feasible in the light of uncertain durations of some processes. We present a fast algorithm for Dynamic Controllability. We also note a correspondence between the reduction steps in the algorithm and the operations involved in converting the projections to dispatchable form. This has implications for the complexity for sparse networks. © 2014 Springer International Publishing. Source

Fleck B.,NASA | Couvidat S.,Stanford University | Straus T.,National institute for astrophysics
Solar Physics | Year: 2011

The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe i absorption line at 6173 Å. We use the output of a high-resolution, 3D, time-dependent, radiation-hydrodynamic simulation based on the CO5BOLD code to calculate profiles F(λ,x,y,t) for the Fe i 6173 Å line. The emerging profiles F(λ,x,y,t) are multiplied by a representative set of HMI filter-transmission profiles Ri(λ, 1≤i≤6) and filtergrams Ii(x,y,t; 1≤i≤6) are constructed for six wavelengths. Doppler velocities VHMI(x,y,t) are determined from these filtergrams using a simplified version of the HMI pipeline. The Doppler velocities are correlated with the original velocities in the simulated atmosphere. The cross-correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal is formed rather low in the solar atmosphere. The same analysis is performed for the SOHO/MDI Ni i line at 6768 Å. The MDI Doppler signal is formed slightly higher at around 125 km. Taking into account the limited spatial resolution of the instruments, the apparent formation height of both the HMI and MDI Doppler signal increases by 40 to 50 km. We also study how uncertainties in the HMI filter-transmission profiles affect the calculated velocities. © 2011 Springer Science+Business Media B.V. Source

Mishchenko M.I.,NASA
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2013

First-principle analysis of the functional design of a well-collimated radiometer (WCR) reveals that in general, this instrument does not record the instantaneous directional flow of electromagnetic energy. Only in special cases can a sequence of measurements with a WCR yield the magnitude and direction of the local time-averaged Poynting vector. Our analysis demonstrates that it is imperative to clearly formulate the physical nature of the actual measurement afforded by a directional radiometer rather than presume desirable measurement capabilities. Only then can the directional radiometer be considered a legitimate part of physically based remote sensing and radiation-budget applications. We also emphasize the need for a better understanding of the nature of measurements with panoramic radiometers. © 2013. Source

Mackowski D.W.,Auburn University | Mishchenko M.I.,NASA
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2013

The exact multiple sphere superposition method is used to calculate the coherent and incoherent contributions to the ensemble-averaged electric field amplitude and Poynting vector in systems of randomly positioned nonabsorbing spherical particles. The target systems consist of cylindrical volumes, with radius several times larger than length, containing spheres with positional configurations generated by a Monte Carlo sampling method. Spatially dependent values for coherent electric field amplitude, coherent energy flux, and diffuse energy flux, are calculated by averaging of exact local field and flux values over multiple configurations and over spatially independent directions for fixed target geometry, sphere properties, and sphere volume fraction. Our results reveal exponential attenuation of the coherent field and the coherent energy flux inside the particulate layer and thereby further corroborate the general methodology of the microphysical radiative transfer theory. An effective medium model based on plane wave transmission and reflection by a plane layer is used to model the dependence of the coherent electric field on particle packing density. The effective attenuation coefficient of the random medium, computed from the direct simulations, is found to agree closely with effective medium theories and with measurements. In addition, the simulation results reveal the presence of a counter-propagating component to the coherent field, which arises due to the internal reflection of the main coherent field component by the target boundary. The characteristics of the diffuse flux are compared to, and found to be consistent with, a model based on the diffusion approximation of the radiative transfer theory. © 2013 Elsevier Ltd. Source

Arendt R.G.,NASA
Astronomical Journal | Year: 2014

Re-examination of the Cosmic Background Explorer Diffuse Infrared Background Experiment (DIRBE) data reveals the thermal emission of several comet dust trails. The dust trails of 1P/Halley, 169P/NEAT, and 3200 Phaethon have not been previously reported. The known trails of 2P/Encke and 73P/Schwassmann-Wachmann 3 are also seen. The dust trails have 12 and 25 μm surface brightnesses of <0.1 and <0.15 MJy sr-1, respectively, which is <1% of the zodiacal light intensity. The trails are very difficult to see in any single daily image of the sky, but are evident as rapidly moving linear features in movies of the DIRBE data. Some trails are clearest when crossing through the orbital plane of the parent comet, but others are best seen at high ecliptic latitudes as the Earth passes over or under the dust trail. All these comets have known associations with meteor showers. This reexamination also reveals 1 additional comet and 13 additional asteroids that had not previously been recognized in the DIRBE data. © 2014. The American Astronomical Society. All rights reserved. Source

Norsk P.,Universities Space Research Association | Norsk P.,NASA
European Journal of Applied Physiology | Year: 2014

During weightlessness, blood and fluids are immediately shifted from the lower to the upper body segments, and within the initial 2 weeks of spaceflight, brachial diastolic arterial pressure is reduced by 5 mmHg and even more so by some 10 mmHg from the first to the sixth month of flight. Blood pressure thus adapts in space to a level very similar to that of being supine on the ground. At the same time, stroke volume and cardiac output are increased and systemic vascular resistance decreased, whereas sympathetic nerve activity is kept surprisingly high and similar to when ground-based upright seated. This was not predicted from simulation models and indicates that dilatation of the arteriolar resistance vessels is caused by mechanisms other than a baroreflex-induced decrease in sympathetic nervous activity. Results of baroreflex studies in space indicate that compared to being ground-based supine, the carotid (vagal)-cardiac interaction is reduced and sympathetic nerve activity, heart rate and systemic vascular resistance response more pronounced during baroreflex inhibition by lower body negative pressure. The future challenge is to identify which spaceflight mechanism induces peripheral arteriolar dilatation, which could explain the decrease in blood pressure, the high sympathetic nerve activity and associated cardiovascular changes. It is also a challenge to determine the cardiovascular risk profile of astronauts during future long-duration deep space missions. © 2014 Springer-Verlag Berlin Heidelberg. Source

Sengupta S.,Indian Institute of Astrophysics | Marley M.S.,NASA
Astrophysical Journal Letters | Year: 2010

Light scattering by atmospheric dust particles is responsible for the polarization observed in some L dwarfs. Whether this polarization arises from an inhomogeneous distribution of dust across the disk or an oblate shape induced by rotation remains unclear. Here, we argue that the latter case is plausible and, for many L dwarfs, the more likely one. Furthermore, evolutionary models of mature field L dwarfs predict surface gravities ranging from about 200 to 2500 m s-2 (corresponding to masses of ∼15-70 MJupiter). Yet comparison of observed spectra to available synthetic spectra often does not permit more precise determination of the surface gravity of individual field L dwarfs, leading to important uncertainties in their properties. Since rotationally induced non-sphericity, which gives rise to non-zero disk-integrated polarization, is more pronounced at lower gravities, polarization is a promising low gravity indicator. Here, we combine a rigorous multiple scattering analysis with a self-consistent cloudy atmospheric model and observationally inferred rotational velocities and find that the observed optical polarization can be explained if the surface gravity of the polarized objects is about 300 m s-2 or less, potentially providing a new method for constraining L dwarf masses. Source

Marley M.S.,NASA | Saumon D.,Los Alamos National Laboratory | Goldblatt C.,University of Washington
Astrophysical Journal Letters | Year: 2010

One mechanism suggested for the L to T dwarf spectral type transition is the appearance of relatively cloud-free regions across the disk of brown dwarfs as they cool. The existence of partly cloudy regions has been supported by evidence for variability in dwarfs in the late L to early T spectral range, but no self-consistent atmosphere models of such partly cloudy objects have yet been constructed. Here, we present a new approach for consistently modeling partly cloudy brown dwarfs and giant planets.We find that even a small fraction of cloud holes dramatically alter the atmospheric thermal profile, spectra, and photometric colors of a given object. With decreasing cloudiness objects briskly become bluer in J - K and brighten in J band, as is observed at the L/T transition. Model spectra of partly cloudy objects are similar to our models with globally homogenous, but thinner, clouds. Hence, spectra alone may not be sufficient to distinguish partial cloudiness although variability and polarization measurements are potential observational signatures. Finally, we note that partial cloud cover may be an alternative explanation for the blue L dwarfs. © 2010. The American Astronomical Society. All rights reserved. Source

Woodard S.E.,NASA
IEEE Transactions on Instrumentation and Measurement | Year: 2010

Closed circuits have been used in electrical systems since Alessandro Volta's invention of the modern battery in 1800 made the first closed electrical circuits possible. Even though a capacitor in a circuit could be considered as an open circuit, its use requires electrical connections to a circuit. This paper reports successful measurement of fluid level using a self-resonating planar pattern of electrically conductive material that is an open-circuit single-component sans electrical connections that is wirelessly powered using external oscillating magnetic fields. The circuit responded with its own magnetic harmonic that changed monotonically with the fluid level. The same external antenna received the circuit response providing a means of interrogating the sensor. By eliminating electrical connections, there is no single point that, if damaged, prevents the circuit from being powered and interrogated, thus, eliminating a key failure mode of electrical devices. Sensors used for the initial fluid-level measurements have portions of their circuits severed from the other portions. The damaged sensors were then successfully used to measure the fluid level. Although the results for fluid-level measurement are presented herein, theoretically, many electrical devices whose functions depend on the use of electric fields, magnetic fields or resistance could be developed as open circuits. The applications are far-reaching, from safer damage-resilient self-sensing human-implanted medical sensors to applications with vehicle systems, buildings, food processing and storage, package tamper detection and other measurements that can be related to capacitance, inductance or resistance. Furthermore, this work lays the foundation for developing other electrical components that can be designed without the use of closed circuits or electrical connections. © 2006 IEEE. Source

Toon O.B.,University of Colorado at Boulder | Segura T.,Northrop Grumman | Zahnle K.,NASA
Annual Review of Earth and Planetary Sciences | Year: 2010

We explore the role of large impacts in creating the Martian valley networks. Recent dating shows that some large impact basins are contemporaneous with the valley networks. The mass deposited (and volatiles released) by impacts is large, and comparable with the mass from the Tharsis volcanic construct. Steam atmospheres formed after large impacts can produce more than 600 m of rainfall, followed by rainfall from water-vapor greenhouse atmospheres, and snowmelt. The erosion rates from impacts that created the currently visible craters are somewhat less than the erosion rates suggested for the Noachian (4.2 to 3.82 Gya). There are several possible explanations for this difference, and it is possible that erosion rates are overestimated because the burial of small craters by global debris layers from impacts has not been considered. Rainfall after the Noachian was low because the impact rate and CO2 pressure declined. We suggest tests of the hypothesis that impacts caused the river valleys. Copyright © 2010 by Annual Reviews. All rights reserved. Source

Hathaway D.H.,NASA
Living Reviews in Solar Physics | Year: 2010

The Solar Cycle is reviewed. The 11-year cycle of solar activity is characterized by the rise and fall in the numbers and surface area of sunspots. We examine a number of other solar activity indicators including the 10.7 cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores that vary in association with the sunspots. We examine the characteristics of individual solar cycles including their maxima and minima, cycle periods and amplitudes, cycle shape, and the nature of active latitudes, hemispheres, and longitudes. We examine long-term variability including the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double peaked maxima. We conclude with an examination of prediction techniques for the solar cycle. Source

Dwek E.,NASA | Krennrich F.,Iowa State University
Astroparticle Physics | Year: 2013

The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy c-rays which are attenuated en route to Earth by pair producing γ- γ interactions with the EBL and CMB. The EBL affects the spectrum of the sources, predominantly blazars, in the ∼10 GeV-10 TeV energy regime. Knowledge of the EBL intensity and spectrum will allow the determination of the intrinsic blazar spectrum in a crucial energy regime that can be used to test particle acceleration mechanisms and very high energy (VHE) γ-ray production models. Conversely, knowledge of the intrinsic c-ray spectrum and the detection of blazars at increasingly higher redshifts will set strong limits on the EBL and its evolution. This paper reviews the latest developments in the determination of the EBL and its impact on the current understanding of the origin and production mechanisms of c-rays in blazars, and on energy releases in the universe. The review concludes with a summary and future directions in Cherenkov Telescope Array techniques and in infrared ground-based and space observatories that will greatly improve our knowledge of the EBL and the origin and production of very high energy c-rays. © 2012 Elsevier B.V. All rights reserved. Source

Koshak W.J.,NASA | Solakiewicz R.J.,Chicago State University
Journal of Atmospheric and Oceanic Technology | Year: 2011

A retrieval method is introduced for estimating the fraction of ground flashes in a set of N flashes observed from either a low earth-orbiting or geostationary satellite lightning imager. The methodology exploits the fact that mean optical characteristics of ground and cloud flashes differ, and hence a properly posed equation set for mean conditions of a set ofN observed flashes can be mathematically inverted to estimate the ground flash fraction (and hence the cloud flash-to-ground flash ratio). Explicit analytic expressions for the retrieval errors are derived, and numerical tests of the retrieval method are provided to quantify retrieval accuracy. It has been found that the retrieval method works best when only one optimum optical parameter is used (the singlecharacteristic solution approach) rather than a mixture of optical parameters (the multiple-characteristic solution approach); that is, the suboptimum optical parameters in the mix degrade retrieval accuracy. Since the retrieval method uses conterminous United States (CONUS)-averaged values of the lightning optical measurements, retrieval errors tend to be smallest in geographical regions whose specific mean lightning optical measurements are closest to the CONUS mean values. The rms ground flash fraction retrieval errors for 52 widely distributed regions across CONUS ranged from as low as 0.061 to 0.111, depending on the true ground flash fraction sought. © 2011 American Meteorological Society. Source

Koshak W.J.,NASA
Journal of Atmospheric and Oceanic Technology | Year: 2011

A Bayesian inversion method is introduced for retrieving the fraction of ground flashes in a set of flashes observed from a (low earth orbiting or geostationary) satellite lightning imager. The method employs a constrained mixed exponential distribution model to describe the lightning optical measurements. Because the method also retrieves certain population statistics of ground and cloud flash optical properties, the method can be applied to an arbitrary geographical region, including those regions where the lightning optical statistics either are not known or are difficult to obtain. The approach is tested by performing simulated retrievals, and retrieval error statistics are provided. A first-attempt retrieval of the global geographical distribution of ground flash fraction is obtained using the 5-yr Optical Transient Detector (OTD) dataset; the spatially averaged ground flash fraction over the global-scale domain studied was 0.151 with a standard deviation of 0.081. The ability to retrieve ground flash fraction has important benefits to the atmospheric chemistry community. For example, using the method to partition the existing OTD/Lightning Imaging Sensor (LIS) satellite global lightning climatology into separate ground and cloud flash climatologies would improve estimates of regional and global lightning nitrogen oxides (NOx) production; this, in turn, would improve both regional air quality and global chemistry/climate model predictions. © 2011 American Meteorological Society. Source

Mishchenko M.I.,NASA | Dlugach J.M.,Ukrainian Academy of Sciences | Mackowski D.W.,Auburn University
Optics Letters | Year: 2011

The numerically exact superposition T-matrix method is used to compute the scattering cross sections and the Stokes scattering matrix for polydisperse spherical particles covered with a large number of much smaller grains. We show that the optical effect of the presence of microscopic dust on the surfaces of wavelength-sized, weakly absorbing particles is much less significant than that of a major overall asphericity of the particle shape. © 2011 Optical Society of America. Source

Stahl H.P.,NASA
Optical Engineering | Year: 2010

Parametric cost models are routinely used to plan missions, compare concepts, and justify technology investments. However, great care is required. There is a lot of confusion and wrong information for space telescopes. Cost estimating relationships based on primary mirror diameter vary by an order of magnitude. Cost estimating relationships based only on mass lack sufficient detail to support concept analysis and can lead to inaccurate conclusions by encouraging excessively complex and technologically immature solutions. Similarly, using ground-based models leads to incorrect conclusions. This work surveys current and historical published cost models for space telescopes while attempting to interpret them in a common logical framework to enable a systematic intercomparison. © 2010 Society of Photo-Optical Instrumentation. Source

MacKowski D.W.,Auburn University | Mishchenko M.I.,NASA
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

The conventional orientation-averaging procedure developed in the framework of the superposition T-matrix approach is generalized to include the case of illumination by a Gaussian beam (GB). The resulting computer code is parallelized and used to perform extensive numerically exact calculations of electromagnetic scattering by volumes of discrete random medium consisting of monodisperse spherical particles. The size parameters of the scattering volumes are 40, 50, and 60, while their packing density is fixed at 5%. We demonstrate that all scattering patterns observed in the far-field zone of a random multisphere target and their evolution with decreasing width of the incident GB can be interpreted in terms of idealized theoretical concepts such as forward-scattering interference, coherent backscattering (CB), and diffuse multiple scattering. It is shown that the increasing violation of electromagnetic reciprocity with decreasing GB width suppresses and eventually eradicates all observable manifestations of CB. This result supplements the previous demonstration of the effects of broken reciprocity in the case of magneto-optically active particles subjected to an external magnetic field. © 2011 The American Physical Society. Source

Mishchenko M.I.,NASA | MacKowski D.W.,Auburn University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We analyze the asymptotic behavior of the cross-polarized enhancement factor in the framework of the standard low-packing-density theory of coherent backscattering by discrete random media composed of spherically symmetric particles. It is shown that if the particles are strongly absorbing or if the smallest optical dimension of the particulate medium (i.e., the optical thickness of a plane-parallel slab or the optical diameter of a spherically symmetric volume) approaches zero, then the cross-polarized enhancement factor tends to its upper-limit value 2. This theoretical prediction is illustrated using direct computer solutions of the Maxwell equations for spherical volumes of discrete random medium. © 2011 American Physical Society. Source

Pak C.-G.,NASA
AIAA Journal | Year: 2016

A new two-step theory is investigated for predicting the deflection and slope of an entire structure using measured strain at discrete locations. In the first step, a measured strain is fitted using a piecewise least-squares curve fitting method together with the cubic spline technique. These fitted strains are integrated twice to obtain deflection data along the optical fibers. In the second step, computed deflection along the optical fibers is combined with a finiteelement model of the structure in order to interpolate and extrapolate the deflection and slope of the entire structure through the use of the System Equivalent Reduction and Expansion Process. The theory is first validated on a computational model, a cantilevered rectangular plate wing. The theory is then applied to test data from a cantilevered swept-plate wing model. Computed results are compared with finite-element results, results using another strain-based method, and photogrammetry data. In general, excellent matching between the target and computed values are accomplished in this study. © Copyright 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source

Allen S.W.,Kavli Institute for Particle Astrophysics and Cosmology | Allen S.W.,SLAC | Evrard A.E.,University of Michigan | Mantz A.B.,NASA
Annual Review of Astronomy and Astrophysics | Year: 2011

Studies of galaxy clusters have proved crucial in helping to establish the standard model of cosmology, with a Universe dominated by dark matter and dark energy. A theoretical basis that describes clusters as massive, multicomponent, quasi-equilibrium systems is growing in its capability to interpret multiwavelength observations of expanding scope and sensitivity. We review current cosmological results, including contributions to fundamental physics, obtained from observations of galaxy clusters. These results are consistent with and complementary to those from other methods. We highlight several areas of opportunity for the next few years, and emphasize the need for accurate modeling of survey selection and sources of systematic error. Capitalizing on these opportunities will require a multiwavelength approach and the application of rigorous statistical frameworks, utilizing the combined strengths of observers, simulators, and theorists. © 2011 by Annual Reviews. All rights reserved. Source

Hartwig J.,NASA | Darr S.,University of Florida
Applied Thermal Engineering | Year: 2014

This paper presents the influential factors which govern screen selection for liquid acquisition devices (LADs) operating in microgravity conditions for future in-space cryogenic propulsion engines and cryogenic propellant depots. Space flight requirements, which include mass flow rate, acceleration level and direction, and thermal environment, dictate screen selection for a particular mission. The five influential factors include bubble point pressure, flow-through-screen pressure drop, wicking rate, screen compliance, and material compatibility. Governing equations and analytical models for these parameters are developed from first principles. A comprehensive survey of the historical data on coarser LAD meshes over four decades of work is conducted, and liquid hydrogen data for finer Dutch Twill meshes (325 × 2300, 450 × 2750, 510 × 3600) from recently concluded experiments is also presented to validate analytical models. Each of these parameters is measurable from ground based tests, making it facile to predict flight system performance. Therefore analytical models in this paper will be valuable for future LAD designs for both cryogenic and storable propulsion systems. Additionally, analysis will be given on the impact of the factors on liquid hydrogen systems. Source

Lahoz W.A.,NILU | Lahoz W.A.,Meteo - France | De Lannoy G.J.M.,NASA
Surveys in Geophysics | Year: 2014

This paper reviews the conceptual problems limiting our current knowledge of the hydrological cycle over land. We start from the premise that to understand the hydrological cycle we need to make observations and develop dynamic models that encapsulate our understanding. Yet, neither the observations nor the models could give a complete picture of the hydrological cycle. Data assimilation combines observational and model information and adds value to both the model and the observations, yielding increasingly consistent and complete estimates of hydrological components. In this review paper we provide a historical perspective of conceptual problems and discuss state-of-the-art hydrological observing, modelling and data assimilation systems. © 2013 The Author(s). Source

Frey H.,NASA
Special Paper of the Geological Society of America | Year: 2011

Lunar topographic and model crustal thickness data provide evidence for large basins on the Moon not previously recognized by photogeologic mapping. The number of these basins larger than 300 km diameter suggests that the total population may be 2-3 times greater than previously thought. Some previously proposed named basins have little to no basin-like topographic character; most of those that do also have pronounced crustal thickness signatures in the form of circular thin areas (CTAs). There also exist CTAs that lack a pronounced quasi-circular depression (QCD) signature in the available topographic data. Newly recognized candidate basins contribute signifi cantly to a total population in which there are more large basins on the lunar farside than on the nearside. The North Pole region appears remarkably devoid of large basins, with only three larger than 300 km diameter and only one at <70°N. Disruption of the topographic and/or crustal thickness structure can be used to establish overlap relationships between QCDs and CTAs and thus relative ages (and a relative local stratigraphy) for those features not visible in images. If large basins have the ability to signifi cantly modify the surface out to twice their diameter, there are likely no parts of the Moon that remain unaffected by the total population shown here. If such effects extend out to only 1.5 basin diameters, there may be 1.8 million km2 of lunar surface where rocks predating most basin formation may still exist. The early history of the Moon likely involved much greater large-diameter impact cratering than previously thought, and therefore much greater global mixing and redistribution of surface materials. Early lunar stratigraphy is likely far more complex than previously appreciated. © 2011 The Geological Society of America. All rights reserved. Source

Thejappa G.,University of Maryland University College | MacDowall R.J.,NASA | Bergamo M.,University of Maryland University College | Papadopoulos K.,University of Maryland University College
Astrophysical Journal Letters | Year: 2012

We present observational evidence for the oscillating two stream instability (OTSI) and spatial collapse of Langmuir waves in the source region of a solar typeIII radio burst. High time resolution observations from the STEREO A spacecraft show that Langmuir waves excited by the electron beam occur as isolated field structures with short durations 3.2ms and with high intensities exceeding the strong turbulence thresholds. These short duration events are identified as the envelope solitons which have collapsed to spatial scales of a few hundred Debye lengths. The spectra of these wave packets contain an intense peak and two sidebands, corresponding to beam-resonant Langmuir waves, and down-shifted and up-shifted daughter Langmuir waves, respectively, and low-frequency enhancements below a few hundred Hz. The frequencies and wave numbers of these spectral components satisfy the resonance conditions of the OTSI. The observed high intensities, short scale lengths, sideband spectral structures, and low-frequency enhancements strongly suggest that the OTSI and spatial collapse of Langmuir waves probably control the nonlinear beam-plasma interactions in typeIII radio bursts. © 2012 The American Astronomical Society. All rights reserved. Source

Hathaway D.H.,NASA
Astrophysical Journal Letters | Year: 2012

Supergranules are convection cells seen at the Sun's surface as a space filling pattern of horizontal flows. While typical supergranules have diameters of about 35 Mm, they exhibit a broad spectrum of sizes from ∼10 Mm to ∼100 Mm. Here we show that supergranules of different sizes can be used to probe the rotation rate in the Sun's outer convection zone. We find that the equatorial rotation rate as a function of depth as measured by global helioseismology matches the equatorial rotation as a function of wavelength for the supergranules. This suggests that supergranules are advected by flows at depths equal to their wavelengths and thus can be used to probe flows at those depths. The supergranule rotation profiles show that the surface shear layer, through which the rotation rate increases inward, extends to depths of ∼50 Mm and to latitudes of at least 70°. Typical supergranules are well observed at high latitudes and have a range of sizes that extend to greater depths than those typically available for measuring subsurface flows with local helioseismology. These characteristics indicate that probing the solar convection zone dynamics with supergranules can complement the results of helioseismology. © Copyright is not claimed for this article. All rights reserved. Source

The International Satellite Cloud Climatology Project (ISCCP) provides a multidecadal and global description of cloud properties that are often grouped into joint histograms of column visible optical depth t and effective cloud-top pressure Ptop. It has not been possible until recently to know the actual distributions of hydrometeor layers within the ISCCP Ptop-τ bins. Distributions of hydrometeor layers within the ISCCP Ptop-τ conditional probability space using measurements from the CloudSat Cloud Profiling Radar and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar within two 40° × 40° regions in the eastern and western equatorial Pacific over a 2-yr period are examined. With the exception of thin cirrus and stratocumulus, the authors show that of the Ptop-τ types that are commonly analyzed, none of the types contain unique distributions of geometrically defined layer types but tend to be populated by diverse sets of hydrometeor layers whose bulk profile properties conspire to render specific radiative signatures when interpreted by two-channel visible and IR sensors from space. In comparing the geometric distribution of cloud layers for common Ptop-τ types, it is found that the ISCCP Cirrostratus, Deep Convection, and Stratocumulus types appear to have been drawn from a common geometric distribution of hydrometeor layers. The other six common ISCCP Ptop-τ types do not share this feature. The authors can confidently reject an assumption that even though they have common top-of-atmosphere radiative signatures, they do not appear to share a common distribution of cloud layers and therefore are likely to have significantly different radiative heating profiles and different surface radiative forcing even though their top-of-atmosphere radiative signatures are similar. © 2013 American Meteorological Society. Source

Sharp Z.D.,University of New Mexico | Draper D.S.,NASA
Earth and Planetary Science Letters | Year: 2013

The Cl, Br and I contents of Earth are depleted by a factor of 10 relative to predicted values from chondritic and solar abundances. Possible explanations for the apparent discrepancy include (1) unrecognized sequestration of Cl in the core, (2) a much higher nebular volatility than normally presumed or (3) a preferential loss of the heavy halogens during planetary accretion. We tested the first assumption by conducting high pressure-temperature equilibration experiments between silicate and metal. At 15GPa and 1900°C, the DCl(metal-silicate) value for Cl is less than 0.007, indicating that the core is not a significant reservoir for Cl. The concentration of Cl in all chondritic classes follows a depletion trend very similar to that of Na and Mn, arguing against a low condensation temperature for Cl. Instead, we propose that the depletion of the heavy halogens is due to their unique hydrophilic behavior. Almost half of Earth's Cl and Br inventory resides in the ocean and evaporites, demonstrating the unique affinity for aqueous solutions for these elements. During planetary accretion, there would have been a strong sequestration of halogens into the crustal reservoir. 'Collisional erosion' during planetary accretion provides a mechanism that would uniquely strip the heavy halogens out of an accreting Earth. Had such loss not occurred, the salinity of the oceans would be 10× the present value, and complex life would probably never have evolved. © 2013 Elsevier B.V. Source

Hathaway D.H.,NASA | Rightmire L.,University of Alabama in Huntsville
Astrophysical Journal | Year: 2011

We measure the axisymmetric transport of magnetic flux on the Sun by cross-correlating narrow strips of data from line-of-sight magnetograms obtained at a 96 minute cadence by the MDI instrument on the ESA/NASA SOHO spacecraft and then averaging the flow measurements over each synodic rotation of the Sun. Our measurements indicate that the axisymmetric flows vary systematically over the solar cycle. The differential rotation is weaker at maximum than at minimum. The meridional flow is faster at minimum and slower at maximum. The meridional flow speed on the approach to the Cycle 23/24 minimum was substantially faster than it was at the Cycle 22/23 minimum. The average latitudinal profile is largely a simple sinusoid that extends to the poles and peaks at about 35° latitude. As the cycle progresses, a pattern of inflows toward the sunspot zones develops and moves equatorward in step with the sunspot zones. These inflows are accompanied by the torsional oscillations. This association is consistent with the effects of the Coriolis force acting on the inflows. The equatorward motions associated with these inflows are identified as the source of the decrease in net poleward flow at cycle maxima. We also find polar countercells (equatorward flow at high latitudes) in the south from 1996 to 2000 and in the north from 2002 to 2010. We show that these measurements of the flows are not affected by the nonaxisymmetric diffusive motions produced by supergranulation. © 2011. The American Astronomical Society. All rights reserved. Source

Allamandola L.J.,NASA
EAS Publications Series | Year: 2011

In dense molecular clouds, the birthplace of stars and planets, interstellar atoms and molecules freeze onto extremely cold dust and ice particles. These ices are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of astrobiological interest. Eventually, these rain down on primordial planets where they take part in the young chemistry on these new worlds. Although the IR spectroscopy and energetic processing of interstellar ice analogs have been studied for nearly 30 years, similar studies of PAH containing ices have only just begun. This paper presents recent results from laboratory studies on the vacuum ultraviolet (VUV) photochemistry of PAHs in water ice at low temperatures to assess the roles they play in the photochemical evolution of interstellar ices and their relevance to astrobiology. A number of "surprises" were found in these studies on PAH containing water-rich ices, indicating that PAHs likely play very important, unexpected roles in cosmic ice chemistry, physics and astrobiology. © EAS, EDP Sciences 2011. Source

Rogers L.A.,Massachusetts Institute of Technology | Bodenheimer P.,University of California at Santa Cruz | Lissauer J.J.,NASA | Seager S.,Massachusetts Institute of Technology
Astrophysical Journal | Year: 2011

Kepler has found hundreds of Neptune-size (2-6 R ⊕) planet candidates within 0.5 AU of their stars. The nature of the vast majority of these planets is not known because their masses have not been measured. Using theoretical models of planet formation, evolution, and structure, we explore the range of minimum plausible masses for low-density exo-Neptunes. We focus on highly irradiated planets with T eq ≥ 500K. We consider two separate formation pathways for low-mass planets with voluminous atmospheres of light gases: core-nucleated accretion and outgassing of hydrogen from dissociated ices. We show that Neptune-size planets at T eq = 500K with masses as small as a few times that of Earth can plausibly be formed by core-nucleated accretion coupled with subsequent inward migration. We also derive a limiting low-density mass-radius relation for rocky planets with outgassed hydrogen envelopes but no surface water. Rocky planets with outgassed hydrogen envelopes typically have computed radii well below 3 R ⊕. For both planets with H/He envelopes from core-nucleated accretion and planets with outgassed hydrogen envelopes, we employ planet interior models to map the range of planet mass-envelope mass-equilibrium temperature parameter space that is consistent with Neptune-size planet radii. Atmospheric mass loss mediates which corners of this parameter space are populated by actual planets and ultimately governs the minimum plausible mass at a specified transit radius. We find that Kepler's 2-6 R ⊕ planet candidates at T eq = 500-1000K could potentially have masses ≲ 4 M ⊕. Although our quantitative results depend on several assumptions, our qualitative finding that warm Neptune-size planets can have masses substantially smaller than those given by interpolating the masses and radii of planets within our Solar System is robust. © 2011. The American Astronomical Society. All rights reserved. Source

Madhusudhan N.,Princeton University | Burrows A.,Princeton University | Currie T.,NASA
Astrophysical Journal | Year: 2011

We have generated an extensive new suite of massive giant planet atmosphere models and used it to obtain fits to photometric data for the planets HR 8799b, c, and d. We consider a wide range of cloudy and cloud-free models. The cloudy models incorporate different geometrical and optical thicknesses, modal particle sizes, and metallicities. For each planet and set of cloud parameters, we explore grids in gravity and effective temperature, with which we determine constraints on the planet's mass and age. Our new models yield statistically significant fits to the data, and conclusively confirm that the HR 8799 planets have much thicker clouds than those required to explain data for typical L and T dwarfs. Both models with (1) physically thick forsterite clouds and a 60 μm modal particle size and (2) clouds made of 1 μm sized pure iron droplets and 1% supersaturation fit the data. Current data are insufficient to accurately constrain the microscopic cloud properties, such as composition and particle size. The range of best-estimated masses for HR 8799b, HR 8799c, and HR 8799d conservatively span 2-12 MJ , 6-13 MJ , and 3-11 M J , respectively, and imply coeval ages between ∼10 and ∼150Myr, consistent with previously reported stellar ages. The best-fit temperatures and gravities are slightly lower than values obtained by Currie etal. using even thicker cloud models. Finally, we use these models to predict the near-to-mid-IR colors of soon-to-be imaged planets. Our models predict that planet-mass objects follow a locus in some near-to-mid-IR color-magnitude diagrams that is clearly separable from the standard L/T dwarf locus for field brown dwarfs. © 2011. The American Astronomical Society. All rights reserved.. Source

Xu Dr. K.-M.,NASA | Cheng A.,Science Systems And Applications Inc.
Journal of Climate | Year: 2013

The eastern Pacific is a climatologically important region. Conventional coupled atmosphere-ocean general circulation models produce positive sea surface temperature biases of 2-5 K in this region because of insufficient stratocumulus clouds. In this study, a global multiscale modeling framework (MMF), which replaces traditional cloud parameterizations with a 2D cloud-resolving model (CRM) in each atmospheric column, is used to examine the seasonal variations of this Pacific region. The CRM component contains an advanced third-order turbulence closure, helping it to better simulate boundary layer turbulence and lowlevel clouds. Compared to available satellite observations of cloud amount, liquid water path, cloud radiative effects, and precipitation, this MMF produces realistic seasonal variations of the eastern Pacific region, although there are some disagreements in the exact location of maximum cloudiness centers in the Peruvian region and the intensity of ITCZ precipitation. Analyses of profile- and subcloud-based decoupling measures reveal very small amplitudes of seasonal variations in the decoupling strength in the subtropics except for those regions off the subtropical coasts where the decoupling measures suggest that the boundary layers should be well coupled in all four seasons. In the Peruvian and Californian regions, the seasonal variations of low clouds are related to those in the boundary layer height and the strength of inversion. Factors that influence the boundary layer and the inversion, such as solar incident radiation, subcloud-layer turbulent mixing, and large-scale subsidence, can collectively explain the seasonal variations of low clouds rather than the deepening-warming mechanism of Bretherton and Wyant cited in earlier studies. © 2013 American Meteorological Society. Source

Gu G.,The Interdisciplinary Center | Gu G.,NASA | Adler R.F.,The Interdisciplinary Center
Climate Dynamics | Year: 2013

This study explores how global precipitation and tropospheric water vapor content vary on the interdecadal/long-term time scale during past three decades (1988-2010 for water vapor), in particular to what extent the spatial structures of their variations relate to changes in surface temperature. EOF analyses of satellite-based products indicate that the first two modes of global precipitation and columnar water vapor content anomalies are in general related to the El Niño-Southern oscillation. The spatial patterns of their third modes resemble the corresponding linear fits/trends estimated at each grid point, which roughly represent the interdecadal/long-term changes happening during the same time period. Global mean sea surface temperature (SST) and land surface temperature have increased during the past three decades. However, the water vapor and precipitation patterns of change do not reflect the pattern of warming, in particular in the tropical Pacific basin. Therefore, other mechanisms in addition to global warming likely exist to account for the spatial structures of global precipitation changes during this time period. An EOF analysis of longer-record (1949-2010) SST anomalies within the Pacific basin (60oN-60oS) indicates the existence of a strong climate regime shift around 1998/1999, which might be associated with the Pacific decadal variability (PDV) as suggested in past studies. Analyses indicate that the observed linear changes/trends in both precipitation and tropospheric water vapor during 1988-2010 seem to result from a combined impact of global mean surface warming and the PDV shift. In particular, in the tropical central-eastern Pacific, a band of increases along the equator in both precipitation and water vapor sandwiched by strong decreases south and north of it are likely caused by the opposite effects from global-mean surface warming and PDV-related, La Niña-like cooling in the tropical central-eastern Pacific. This narrow band of precipitation increase could also be considered an evidence for the influence of global mean surface warming. © 2012 Springer-Verlag. Source

Journal of Geophysical Research: Oceans | Year: 2013

A new set of pelagic tide determinations is constructed from seafloor pressure measurements obtained at 151 sites in the deep ocean. To maximize precision of estimated tides, only stations with long time series are used; median time series length is 567 days. Geographical coverage is considerably improved by use of the international tsunami network, but coverage in the Indian Ocean and South Pacific is still weak. As a tool for assessing global ocean tide models, the data set is considerably more reliable than older data sets: the root-mean-square difference with a recent altimetric tide model is approximately 5 mm for the M2 constituent. Precision is sufficiently high to allow secondary effects in altimetric and bottom-pressure tide differences to be studied. The atmospheric tide in bottom pressure is clearly detected at the S1, S2, and T2 frequencies. The altimetric tide model is improved if satellite altimetry is corrected for crustal loading by the atmospheric tide. Models of the solid body tide can also be constrained. The free core-nutation effect in the K1 Love number is easily detected, but the overall estimates are not as accurate as a recent determination with very long baseline interferometry. © 2013. American Geophysical Union. All Rights Reserved. Source

In this study, the annual mean climatology of top of the atmosphere (TOA) shortwave and longwave cloud radiative effects in 12 Atmospheric Model Intercomparison Project (AMIP)-type simulations participating in the CoupledModel Intercomparison Project Phase 5 (CMIP5) is evaluated and investigated using satellite-based observations, with a focus on the tropics. Results show that the CMIP5 AMIPs simulate large-scale regional mean TOA radiative fluxes and cloud radiative forcings (CRFs) well but produce considerably less cloud amount, particularly in the middle and lower troposphere. The good model simulations in tropical means, with multimodel mean biases of -3.6W/m2 for shortwave CRF and -1.0 W/m2 for longwave CRF, are, however, a result of compensating errors over different dynamical regimes. Over the Maritime Continent, most of the models simulate moderately less highcloud fraction, leading to weaker shortwave cooling and longwave warming and a larger net cooling. Over subtropical strong subsidence regimes, most of the CMIP5 models strongly underestimate stratocumulus cloud amount and show considerably weaker local shortwave CRF. Over the transitional trade cumulus regimes, a notable feature is that while at varying amplitudes, most of the CMIP5 models consistently simulate a deeper and drier boundary layer, more moist free troposphere, and more high clouds and, consequently, overestimate shortwave cooling and longwave warming effects there. While most of the CMIP5 models show the same sign as the multimodel mean, there are substantial model spreads, particularly over the tropical deep convective and subtropical strong subsidence regimes. Representing clouds and their TOA radiative effects remains a challenge in the CMIP5 models. © 2012. American Geophysical Union. Source

Wang L.,Louisiana State University | Eldridge J.I.,NASA | Guo S.M.,Louisiana State University
Acta Materialia | Year: 2014

The thermal radiative properties of thermal barrier coatings (TBCs) are becoming more important as the inlet temperatures of advanced gas-turbine engines are continuously being pushed higher in order to improve efficiency. To determine the absorption and scattering coefficients of TBCs, four-flux, two-flux and Kubelka-Munk models were introduced and used to characterize the thermal radiative properties of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings. The results show that the absorption coefficient of YSZ is extremely low for wavelengths <6 μm and the scattering coefficient is high and decreases with increasing wavelength. The obvious deviation of absorption and scattering coefficients obtained by the Kubelka-Munk model from those values calculated by four-flux and two-flux models indicates that surface reflection is an important parameter which cannot be neglected when characterizing the radiative property of the coating. The excellent agreement of predicted reflectance and transmittance spectra by the two-flux and four-flux models for coating thicknesses >200 μm suggests that when the coating thickness is larger than around twice the average scattering distance, the collimated flux can be simply treated as a diffuse flux inside the coating, and thus the two-flux model can be used to determine the absorption and scattering coefficients as a simplification of the four-flux model. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Painemal D.,NASA | Zuidema P.,University of Miami
Atmospheric Chemistry and Physics | Year: 2013

The first aerosol indirect effect (1AIE) is investigated using a combination of in situ and remotely-sensed aircraft (NCAR C-130) observations acquired during VOCALS-REx over the southeast Pacific stratocumulus cloud regime. Satellite analyses have previously identified a high albedo susceptibitility to changes in cloud microphysics and aerosols over this region. The 1AIE was broken down into the product of two independently-estimated terms: the cloud aerosol interaction metric ACI τ Combining double low linτ Na|LWP , and the relative albedo (A) susceptibility S Rτ Combining double low line A/3dτ|LWP, with τ/and N a denoting retrieved cloud optical thickness and in situ aerosol concentration respectively and calculated for fixed intervals of liquid water path (LWP). ACI τ was estimated by combining in situ N/a sampled below the cloud, with τ and LWP derived from, respectively, simultaneous upward-looking broadband irradiance and narrow field-of-view millimeter-wave radiometer measurements, collected at 1 Hz during four eight-hour daytime flights by the C-130 aircraft. ACIτ values were typically large, close to the physical upper limit (0.33), with a modest increase with LWP. The high ACI τ values slightly exceed values reported from many previous in situ airborne studies in pristine marine stratocumulus and reflect the imposition of a LWP constraint and simultaneity of aerosol and cloud measurements. S Rτ increased with LWP and τ, reached a maximum SR τ (0.086) for LWP (τ) of 58 g m-2 (∼14), and decreased slightly thereafter. The 1AIE thus increased with LWP and is comparable to a radiative forcing of-3.2-3.8 W m-2 for a 10% increase in Na, exceeding previously-reported global-range values. The aircraft-derived values are consistent with satellite estimates derived from instantaneous, collocated Clouds and the Earth's Radiant Energy System (CERES) albedo and MOderate resolution Imaging Spectroradiometer (MODIS)-retrieved droplet number concentrations at 50 km resolution. The consistency of the airborne and satellite estimates, despite their independent approaches, differences in observational scales, and retrieval assumptions, is hypothesized to reflect the ideal remote sensing conditions for these homogeneous clouds. We recommend the southeast Pacific for regional model assessments of the first aerosol indirect effect on this basis. This airborne remotely-sensed approach towards quantifying 1AIE should in theory be more robust than in situ calculations because of increased sampling. However, although the technique does not explicitly depend on a remotely-derived cloud droplet number concentration (Nd), the at-times unrealistically-high Nd values suggest more emphasis on accurate airborne radiometric measurements is needed to refine this approach. © 2013 Author(s). Source

Kharangate C.R.,Purdue University | Mudawar I.,Purdue University | Hasan M.M.,NASA
International Journal of Heat and Mass Transfer | Year: 2012

This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a 2.5 mm × 5 mm vertical upflow channel that is heated along its 2.5 mm sidewall downstream of an adiabatic development section. Unlike most prior CHF studies, where the working fluid enters the channel in liquid state, the present study concerns saturated inlet conditions with finite vapor void. Temperature measurements and high-speed video imaging techniques are used to investigate the influence of the inlet vapor void on interfacial behavior at heat fluxes up to CHF as well during the CHF transient. The flow entering the heated portion of the channel consists of a thin liquid layer covering the entire perimeter surrounding a large central vapor core. Just prior to CHF, a fairly continuous wavy vapor layer begins to develop between the liquid layer covering the heated wall and the heated wall itself, resulting in a complex four-layer flow consisting of the liquid layer covering the insulated walls, the central vapor core, the now separated liquid layer adjacent to the heated wall, and the newly formed wavy vapor layer along the heated wall. This behavior in captured in a new separated control-volume-based model that facilities the determination of axial variations of thicknesses and mean velocities of the four layers. Incorporating the results of this model in a modified form of the Interfacial Lift-off CHF Model is shown to provide fairly good predictions of CHF data for mass velocities between 185 and 1600 kg/m 2 s, evidenced by a mean absolute error of 24.52%. © 2011 Elsevier Ltd. All rights reserved. Source

Huang X.,Search for Extraterrestrial Intelligence Institute | Lee T.J.,NASA
Astrophysical Journal | Year: 2011

Recently, we reported ab initio quartic force fields (QFFs) for the cyclic and linear forms of the C3H3 + molecular cation, referred to as c-C3H3 + and l-C 3H3 +. These were computed using high levels of theory. Specifically the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), was used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation were included. In the present study, we use these QFFs to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants for the c- 13CC2H3 +, c-C3H 2D+, c-13CC2H2D + isotoplogues of c-C3H3 +, and the H2CCCD+, HDCCCH+, H2 13CCCH+, H2C13CCH+, and H2CC13CH+ isotopologues of l-C 3H3 +. Improvements in ab intitio methods have now made it possible to identify small molecules in an astronomical observation without the aid of high-resolution experimental data. We also report dipole moment values and show that the above-mentioned cyclic isotopologues have values of 0.094, 0.225, and 0.312 D, respectively, while the l-C3H 3 + isotopologues have values that range between 0.325 and 0.811 D. Thus, it is hoped that the highly accurate spectroscopic constants and data provided herein for the 13C and deuterium isotopologues of the cyclic and linear forms of C3H3 + will enable their identification in astronomical observations from the Herschel Space Observatory, the Stratospheric Observatory for Infrared Astronomy, the Atacama Large Millimeter Array, and in the future, the James Webb Space Telescope. © 2011. The American Astronomical Society. All rights reserved. Source

An analysis of simulated cloud regime transitions along a transect from the subtropical California coast to the tropics for the northern summer season (June-August) is presented in this study. The Community Atmosphere Model, version 5 (CAM5), superparameterized CAM (SPCAM), and an upgraded SPCAM with intermediately prognostic higher-order closure (SPCAM-IPHOC) are used to perform global simulations by imposing climatological sea surface temperature and sea ice distributions. The seasonal-mean properties are compared with recent observations of clouds, radiation, and precipitation and with multimodel intercomparison results. There are qualitative agreements in the characteristics of cloud regimes along the transect among the three models. CAM5 simulates precipitation and shortwave radiative fluxes well but the stratocumulus-to-cumulus transition occurs too close to the coast of California. SPCAM-IPHOC simulates longwave radiative fluxes and precipitable water well, but with systematic biases in shortwave radiative fluxes. The broad, stronger ascending band in SPCAM is related to the large biases in the convective region but the characteristics of the stratocumulus region are still more realistic and the transition occurs slightly farther away from the coast than in CAM5. Even though SPCAM-IPHOC produces the most realistic seasonal-mean transition, it underestimates the mean gradient in low-cloud cover (LCC) across the mean transition location because of an overestimate of LCC in the transition and convective regions that shifts the transition locations farther from the coast. Analysis of two decoupling measures shows consistency in the mean location and the histogram of decoupling locations with those of LCC transition. CAM5, however, lacks such a consistency, suggesting a need for further refinement of its boundary layer cloud parameterization. © 2013 American Meteorological Society. Source

The stress rupture strength of silicon carbide fiber-reinforced silicon carbide (SiC/SiC) composites with a boron nitride (BN) fiber coating decreases with time within the intermediate temperature range of 700-950. °C. Various theories have been proposed to explain the cause of the time-dependent stress rupture strength. The objective of this paper is to investigate the relative significance of the various theories for the time-dependent strength of SiC/SiC composites. This is achieved through the development of a numerically-based progressive failure analysis routine and through the application of the routine to simulate the composite stress rupture tests. The progressive failure routine is a time-marching routine with an iterative loop between a probability of fiber survival equation and a force equilibrium equation within each time step. Failure of the composite is assumed to initiate near a matrix crack and the progression of fiber failures occurs by global load sharing. The probability of survival equation is derived from consideration of the strength of ceramic fibers with randomly occurring and slow growing flaws as well as the mechanical interaction between the fibers and matrix near a matrix crack. The force equilibrium equation follows from the global load sharing presumption. The results of progressive failure analyses of the composite tests suggest that the relationship between time and stress-rupture strength is attributed almost entirely to the slow flaw growth within the fibers. Although other mechanisms may be present, they appear to have only a minor influence on the observed time-dependent behavior. © 2016. Source

Cargill P.J.,Imperial College London | Cargill P.J.,University of St. Andrews | Bradshaw S.J.,Rice University | Klimchuk J.A.,NASA
Astrophysical Journal | Year: 2012

This paper develops the zero-dimensional (0D) hydrodynamic coronal loop model "Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by Klimchuk et al., which studies the plasma response to evolving coronal heating, especially impulsive heating events. The basis of EBTEL is the modeling of mass exchange between the corona and transition region (TR) and chromosphere in response to heating variations, with the key parameter being the ratio of the TR to coronal radiation. We develop new models for this parameter that now include gravitational stratification and a physically motivated approach to radiative cooling. A number of examples are presented, including nanoflares in short and long loops, and a small flare. The new features in EBTEL are important for accurate tracking of, in particular, the density. The 0D results are compared to a 1D hydro code (Hydrad) with generally good agreement. EBTEL is suitable for general use as a tool for (1) quick-look results of loop evolution in response to a given heating function, (2) extensive parameter surveys, and (3) situations where the modeling of hundreds or thousands of elemental loops is needed. A single run takes a few seconds on a contemporary laptop. © 2012. The American Astronomical Society. All rights reserved.. Source

Barth T.,NASA
Theoretical and Computational Fluid Dynamics | Year: 2012

This work considers a new class of finite-volume approximations for scalar and system nonlinear conservation laws with multiple sources of stochastic model parameter uncertainty. The deterministic propagation of model parameter uncertainty is achieved through the introduction of additional stochastic coordinates. Particular attention is given to the construction of specialized piecewise polynomial approximation spaces well suited to the high-order accurate approximation of solution discontinuities in both physical and stochastic dimensions without exhibiting Gibbs-like oscillations characteristic of polynomial approximation. The proposed discretization easily retrofits existing finite-volume CFD codes in use today. Numerical results are presented for inviscid Burgers equation with uncertain initial data as well as the compressible Reynolds-averaged Navier-Stokes equations with uncertain boundary data and turbulence model parameters. © Springer-Verlag 2011. Source

Kashlinsky A.,NASA | Atrio-Barandela F.,University of Salamanca | Ebeling H.,University of Hawaii at Manoa
Astrophysical Journal | Year: 2011

We present new results on the "dark flow" from a measurement of the dipole in the distribution of peculiar velocities of galaxy clusters, applying the methodology proposed and developed by us earlier. Our latest measurement is conducted using new, low-noise 7 yr WMAP data as well as an all-sky sample of X-ray-selected galaxy clusters compiled exclusively from published catalogs. Our analysis of the cosmic microwave background signature of the kinematic Sunyaev-Zel'dovich (SZ) effect finds a statistically significant dipole at the location of galaxy clusters. The residual dipole outside the cluster regions is small, rendering our overall measurement 3σ-4σ significant. The amplitude of the dipole correlates with cluster properties, being larger for the most X-ray luminous clusters, as required if the signal is produced by the SZ effect. Since it is measured at zero monopole, the dipole cannot be due to the thermal SZ effect. Our results are consistent with those obtained earlier by us from 5 yr WMAP data and using a proprietary cluster catalog. In addition, they are robust to quadrupole removal, demonstrating that quadrupole leakage contributes negligibly to the signal. The lower noise of the 7 yr WMAP also allows us, for the first time, to obtain tentative empirical confirmation of our earlier conjecture that the adopted filtering alters the sign of the kinematic SZ (KSZ) effect for realistic clusters and thus of the deduced direction of the flow. The latter is consistent with our earlier measurement in both the amplitude and direction. Assuming the filtering indeed alters the sign of the KSZ effect from the clusters, the direction agrees well also with the results of independent work using galaxies as tracers at lower distances. We make all maps and cluster templates derived by us from public data available to the scientific community to allow independent tests of our method and findings. © 2011. The American Astronomical Society. All rights reserved. Source

Venters T.M.,NASA | Pavlidou V.,California Institute of Technology
Astrophysical Journal | Year: 2011

The spectral shapes of the contributions of different classes of unresolved gamma-ray emitters can provide insight into their relative contributions to the extragalactic gamma-ray background (EGB) and the natures of their spectra atGeV energies. We calculate the spectral shapes of the contributions to the EGB arising from BL Lac objects and flat-spectrum radio quasars assuming blazar spectra can be described as broken power laws. We fit the resulting total blazar spectral shape to the Fermi Large Area Telescope measurements of the EGB, finding that the best-fit shape reproduces well the shape of the Fermi EGB for various break scenarios. We conclude that a scenario in which the contribution of blazars is dominant cannot be excluded on spectral grounds alone, even if spectral breaks are shown to be common among Fermi blazars. We also find that while the observation of a featureless (within uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a single class of contributing unresolved sources with featureless individual spectra, such an observation and the collective spectra of the separate contributing populations determine the ratios of their contributions. As such, a comparison with studies including blazar gamma-ray luminosity functions could have profound implications for the blazar contribution to the EGB, blazar evolution, and blazar gamma-ray spectra and emission. © 2011. The American Astronomical Society. All rights reserved. Source

Orr M.R.,Iowa State University | Krennrich F.,Iowa State University | Dwek E.,NASA
Astrophysical Journal | Year: 2011

Direct measurements of the extragalactic background light (EBL) in the near-IR to mid-IR waveband are extremely difficult due to an overwhelming foreground from the zodiacal light that outshines the faint cosmological diffuse radiation field by more than an order of magnitude. Indirect constraints on the EBL are provided by γ-ray observations of active galactic nuclei. Using the combination of the Fermi Gamma-Ray Space Telescope together with the current generation of ground-based air Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) provides unprecedented sensitivity and spectral coverage for constraining the EBL in the near- to mid-IR. In this paper, we present new limits on the EBL based on the analysis of the broadband spectra of a select set of γ-ray blazars covering 200 MeV to several TeV. The EBL intensity at 15 μm is constrained to be 1.36 ± 0.58 nW m-2 sr -1. We find that the fast evolution and baseline EBL models of Stecker et al., as well as the model of Kneiske et al., predict significantly higher EBL intensities in the mid-IR (15 μm) than is allowed by the constraints derived here. In addition, the model of Franceschini et al. and the fiducial model of Domínguez et al. predict near- to mid-IR ratios smaller than that derived from our analysis. Namely, their intensities in the near-IR are too low while their intensities in the mid-IR are marginally too high. All of the aforementioned models are inconsistent with our analysis at the >3σ level. © 2011. The American Astronomical Society. All rights reserved. Source

Kandula M.,NASA
International Communications in Heat and Mass Transfer | Year: 2012

Experiments are performed concerning frost growth and densification in laminar flow over a flat surface under conditions of constant and variable humidity. The flat plate test specimen is made of aluminum-6031, and has dimensions of 0.3 m × 0.3 m × 6.35. mm. Results for the first variable humidity case are obtained for a plate temperature of 255.4. K, air velocity of 1.77. m/s, air temperature of 295.1. K, and a relative humidity continuously ranging from 81 to 54%. The second variable humidity test case corresponds to plate temperature of 255.4. K, air velocity of 2.44. m/s, air temperature of 291.8. K, and a relative humidity ranging from 66 to 59%. Results for the constant humidity case are obtained for a plate temperature of 263.7. K, air velocity of 1.7. m/s, air temperature of 295. K, and a relative humidity of 71.6%. Comparisons of the data with the author's frost model extended to accommodate variable humidity suggest satisfactory agreement between the theory and the data for both constant and variable humidity. © 2012 Elsevier Ltd. Source

Kawakita H.,Kyoto Sangyo University | Mumma M.J.,NASA
Astrophysical Journal | Year: 2011

Ammonia is a major reservoir of nitrogen atoms in cometary materials. However, detections of ammonia in comets are rare, with several achieved at radio wavelengths. A few more detections were obtained through near-infrared observations (around the 3 μm wavelength region), but moderate relative velocity shifts are required to separate emission lines of cometary ammonia from telluric absorption lines in the 3 μm wavelength region. On the other hand, the amidogen radical (NH2-a photodissociation product of ammonia in the coma) also shows rovibrational emission lines in the 3 μm wavelength region. Thus, gas production rates for ammonia can be determined from the rovibrational emission lines of ammonia (directly) and amidogen radical (indirectly) simultaneously in the near-infrared. In this article, we present new fluorescence excitation models for cometary ammonia and amidogen radical in the near-infrared, and we apply these models to the near-infrared high-dispersion spectra of comet C/2004 Q2 (Machholz) to determine the mixing ratio of ammonia to water in the comet. Based on direct detection of NH 3 lines, the mixing ratio of NH3/H2O is 0.46% ± 0.03% in C/2004 Q2 (Machholz), in agreement with other results. The mixing ratio of ammonia determined from the NH2 observations (0.31%-0.79%) is consistent but has relatively larger error, owing to uncertainty in the photodissociation rates of ammonia. At the present level of accuracy, we confirm that NH3 could be the sole parent of NH 2 in this comet. © 2011. The American Astronomical Society. Source

Dwek E.,NASA | Cherchneff I.,University of Basel
Astrophysical Journal | Year: 2011

Two distinct scenarios for the origin of the ∼4 × 108 M⊙ of dust observed in the high-redshift (z = 6.4) quasar J1148+5251 have been proposed. The first assumes that this galaxy is much younger than the age of the universe at that epoch so that only supernovae (SNe) could have produced this dust. The second scenario assumes a significantly older galactic age, so that the dust could have formed in lower-mass asymptotic giant branch (AGB) stars. Presenting new integral solutions for the chemical evolution of metals and dust in galaxies, we offer a critical evaluation of these two scenarios and observational consequences that can discriminate between the two. We show that AGB stars can produce the inferred mass of dust in this object, however, the final mass of surviving dust depends on the galaxy's star formation history (SFH). In general, SNe cannot produce the observed amount of dust unless the average SN event creates over ∼2 M⊙ of dust in its ejecta. However, special SFHs can be constructed in which SNe can produce the inferred dust mass with a reasonable average dust yield of ∼0.15 M ⊙. The two scenarios propose different origins for the galaxy's spectral energy distribution, different star formation efficiencies and stellar masses, and consequently different comoving number densities of J1148+5251-type hyperluminous infrared (IR) objects. The detection of diagnostic mid-IR fine structure lines and more complete surveys determining the comoving number density of these objects can discriminate between the two scenarios. Source

Daescu D.N.,Portland State University | Todling R.,NASA
Quarterly Journal of the Royal Meteorological Society | Year: 2010

The development of the adjoint of the forecast model and of the adjoint of the data assimilation system (adjoint-DAS) makes feasible the evaluation of the local sensitivity of a model forecast aspect with respect to a large number of parameters in the DAS. In this study it is shown that, by exploiting sensitivity properties that are intrinsic to the analyses derived from a minimization principle, the adjoint-DAS software tools developed at numerical weather prediction centres for observation and background sensitivity may be used to estimate the forecast sensitivity to observation- and background-error covariance parameters and for forecast impact assessment. All-at-once sensitivity to error covariance weighting coefficients and first-order impact estimates are derived as a particular case of the error covariance perturbation analysis. The use of the sensitivity information as a DAS diagnostic tool and for implementing gradient-based error covariance tuning algorithms is illustrated in idealized data assimilation experiments with the Lorenz 40-variable model. Preliminary results of forecast sensitivity to observation- and background-error covariance weight parameters are presented using the fifth-generation NASA Goddard Earth Observing System (GEOS-5) atmospheric DAS and its adjoint developed at the Global Modeling and Assimilation Office. © 2010 Royal Meteorological Society. Source

Hall T.,NASA | Yonekura E.,Columbia University
Journal of Climate | Year: 2013

A statistical-stochastic model of the complete life cycle of North Atlantic (NA) tropical cyclones (TCs) is used to examine the relationship between climate and landfall rates along the North American Atlantic and Gulf Coasts. The model draws on archived data of TCs throughout the North Atlantic to estimate landfall rates at high geographic resolution as a function of the ENSO state and one of two different measures of sea surface temperature (SST): 1) SST averaged over theNAsubtropics and the hurricane season and 2) this SST relative to the seasonal global subtropicalmean SST (termed relSST).Here, the authors focus on SST by holding ENSOto a neutral state. Jackknife uncertainty tests are employed to test the significance of SST and relSST landfall relationships. There are more TC and major hurricane landfalls overall in warm years than cold, using either SST or relSST, primarily due to a basinwide increase in the number of storms. The signal along the coast, however, is complex. Some regions have large and significant sensitivity (e.g., an approximate doubling of annualmajor hurricane landfall probability on Texas from -2 to+2 standard deviations in relSST), while other regions have no significant sensitivity (e.g., the U.S. mid-Atlantic and Northeast coasts). This geographic structure is due to both shifts in the regions of primary TC genesis and shifts in TC propagation. © 2013 American Meteorological Society. Source

Mishchenko M.I.,NASA | Dlugach Z.M.,Ukrainian Academy of Sciences | Zakharova N.T.,Trinnovim LLC
Optics Letters | Year: 2014

The modified unrestricted effective-medium refractive index is defined as one that yields accurate values of a representative set of far-field scattering characteristics (including the scattering matrix) for an object made of randomly heterogeneous materials. We validate the concept of the modified unrestricted effective-medium refractive index by comparing numerically exact superposition T-matrix results for a spherical host randomly filled with a large number of identical small inclusions and Lorenz-Mie results for a homogeneous spherical counterpart. A remarkable quantitative agreement between the superposition T-matrix and Lorenz-Mie scattering matrices over the entire range of scattering angles demonstrates unequivocally that the modified unrestricted effective-medium refractive index is a sound (albeit still phenomenological) concept provided that the size parameter of the inclusions is sufficiently small and their number is sufficiently large. Furthermore, it appears that in cases when the concept of the modified unrestricted effective-medium refractive index works, its actual value is close to that predicted by the Maxwell-Garnett mixing rule. © 2014 Optical Society of America. Source

The Moon maintains large surface temperatures on its illuminated hemisphere and can contribute significant amounts of flux to spatially unresolved thermal infrared (IR) observations of the Earth-Moon system, especially at wavelengths where Earth's atmosphere is absorbing. In this paper we investigate the effects of an unresolved companion on IR observations of Earthlike exoplanets. For an extrasolar twin Earth-Moon system observed at full phase at IR wavelengths, the Moon consistently comprises about 20% of the total signal, approaches 30% of the signal in the 9.6μm ozone band and the 15μm carbon dioxide band, makes up as much as 80% of the signal in the 6.3μm water band, and more than 90% of the signal in the 4.3μm carbon dioxide band. These excesses translate to inferred brightness temperatures for Earth that are too large by 20-40 K and demonstrate that the presence of undetected satellites can have significant impacts on the spectroscopic characterization of exoplanets. The thermal flux contribution from an airless companion depends strongly on phase, implying that observations of exoplanets should be taken when the star-planet-observer angle (i.e., phase angle) is as large as feasibly possible if contributions from companions are to be minimized. We show that, by differencing IR observations of an Earth twin with a companion taken at both gibbous and crescent phases, Moonlike satellites may be detectable by future exoplanet characterization missions for a wide range of system inclinations. © 2011 The American Astronomical Society. All rights reserved. Source

Tripathi D.,Inter-University Center for Astronomy and Astrophysics | Tripathi D.,University of Cambridge | Klimchuk J.A.,NASA | Mason H.E.,University of Cambridge
Astrophysical Journal | Year: 2011

Using data from the Extreme-ultraviolet Imaging Spectrometer aboard Hinode, we have studied the coronal plasma in the core of two active regions. Concentrating on the area between opposite polarity moss, we found emission measure distributions having an approximate power-law form EMT ∝ 2.4 from log T = 5.5 up to a peak at log T = 6.55. We show that the observations compare very favorably with a simple model of nanoflare-heated loop strands. They also appear to be consistent with more sophisticated nanoflare models. However, in the absence of additional constraints, steady heating is also a viable explanation. © 2011. The American Astronomical Society. All rights reserved. Source

Stothers R.B.,NASA
Publications of the Astronomical Society of the Pacific | Year: 2010

Among RR Lyrae stars displaying the Blazhko effect, a few show no period modulation in spite of striking changes in their light amplitudes. This anomalous behavior and the mean period of the affected variables are predicted correctly by the theory of slow convective cycles in the stellar envelope. © 2010. The Astronomical Society of the Pacific. All rights reserved. Source

Kuchner M.J.,NASA | Stark C.C.,University of Maryland University College
Astronomical Journal | Year: 2010

We modeled the three-dimensional structure of the Kuiper Belt (KB) dust cloud at four different dust production rates, incorporating both planet-dust interactions and grain-grain collisions using the collisional grooming algorithm. Simulated images of a model with a face-on optical depth of -10 -4 primarily show an azimufhally symmetric ring at 40-47 AU in submillimeter and infrared wavelengths; this ring is associated with the cold classical KB. For models with lower optical depths (10-6 and 10 -7), synthetic infrared images show that the ring widens and a gap opens in the ring at the location of Neptune; this feature is caused by trapping of dust grains in Neptune's mean motion resonances. At low optical depths, a secondary ring also appears associated with the hole cleared in the center of the disk by Saturn. Our simulations, which incorporate 25 different grain sizes, illustrate that grain-grain collisions are important in sculpting today's KB dust, and probably other aspects of the solar system dust complex; collisions erase all signs of azimuthal asymmetry from the submillimeter image of the disk at every dust level we considered. The model images switch from being dominated by resonantly trapped small grains ("transport dominated") to being dominated by the birth ring ("collision dominated") when the optical depth reaches a critical value of τ - v/c, where v is the local Keplerian speed. © 2010. The American Astronomical Society. All rights reserved. Source

Dorodnitsyn A.V.,NASA
Monthly Notices of the Royal Astronomical Society | Year: 2010

We calculate profiles of spectral lines from an extended outflow from the compact object (a black hole or a neutron star). We assume that the bulk velocity of the flow increases during a short phase of acceleration and then rapidly decreases forming a failed wind. We also study the wind which is only decelerating. We show that depending on the relative strength of the gravitational redshifting line profiles from such winds may be of several types: distorted P-Cygni (emission and blueshifted absorption); W-shaped (absorption-emission-absorption) and inverted P-Cygni (emission-. redshifted absorption). The latter case is expected from accretion flows where the velocity is directed inwards; however, we show that inverted P-Cygni profile can be produced by the failed wind, provided the line is formed within several tens of Schwarzschild radii from the compact object. © 2010 The Author. Journal compilation © 2010 RAS. Source

Garofalo D.,Jet Propulsion Laboratory | Evans D.A.,Massachusetts Institute of Technology | Sambruna R.M.,NASA
Monthly Notices of the Royal Astronomical Society | Year: 2010

Recent work on the engines of active galactic nuclei jets suggests that their power depends strongly and perhaps counter-intuitively on black hole spin. We explore the consequences of this on the radio-loud population of active galactic nuclei and find that the time evolution of the most powerful radio galaxies and radio-loud quasars fits into a picture in which black hole spin varies from retrograde to prograde with respect to the accreting material. Unlike the current view, according to which jet powers decrease in tandem with a global downsizing effect, we argue for a drop in jet power resulting directly from the paucity of retrograde accretion systems at lower redshift z caused by a continuous history of accretion dating back to higher z. In addition, the model provides simple interpretations for the basic spectral features differentiating radio-loud and radio-quiet objects, such as the presence or absence of disc reflection, broadened iron lines and signatures of disc winds. We also briefly describe our models' interpretation of microquasar state transitions. We highlight our result that the most radio-loud and most radio-quiet objects both harbour highly spinning black holes but in retrograde and prograde configurations, respectively. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Datta A.,U.S. Army | Johnson W.,NASA
Journal of the American Helicopter Society | Year: 2011

This paper implements and analyzes a dual-primal iterative substructuring method that is parallel and scalable for the solution of a three-dimensional finite element-based dynamic analysis of helicopter rotor blades. The analysis is developed using isoparametric hexahedral brick finite elements. Particular emphasis is placed on the formulation of the inertial terms that are unique to rotary wing dynamics. The scalability of the solution method is studied using two prototype problems - one for steady hover (symmetric) and one for transient forward flight (nonsymmetric) - both carried out on up to 48 processors with each substructure on a separate processor. In both hover and forward flight, a linear speedup is observed with number of processors, up to the point of substructure optimality. Substructure optimality and the linear speedup range are shown to depend both on the problem size as well as a corner-based global coarse problem selection. An increase in problem size extends the linear speedup range up to a new substructure optimality. A smaller coarse problem selection extends optimality to a greater number of processors. The key conclusion is that a three-dimensional finite element analysis of a helicopter rotor can be carried out in a fully parallel and scalable manner. The careful selection of substructure corner nodes that are used to construct the global coarse problem is key to extending linear speedup to as high a processor number as possible, thus minimizing the solution time for a given problem size. © 2011 The American Helicopter Society. Source

Choi D.S.,NASA | Dundas C.M.,U.S. Geological Survey
Geophysical Research Letters | Year: 2011

We report wind measurements within Martian dust devils observed in plan view from the High Resolution Imaging Science Experiment (HiRISE) orbiting Mars. The central color swath of the HiRISE instrument has three separate charge-coupled devices (CCDs) and color filters that observe the surface in rapid cadence. Active features, such as dust devils, appear in motion when observed by this region of the instrument. Our image animations reveal clear circulatory motion within dust devils that is separate from their translational motion across the Martian surface. Both manual and automated tracking of dust devil clouds reveal tangential winds that approach 20-30 m s -1 in some cases. These winds are sufficient to induce a ∼1% decrease in atmospheric pressure within the dust devil core relative to ambient, facilitating dust lifting by reducing the threshold wind speed for particle elevation. Finally, radial velocity profiles constructed from our automated measurements test the Rankine vortex model for dust devil structure. Our profiles successfully reveal the solid body rotation component in the interior, but fail to conclusively illuminate the profile in the outer regions of the vortex. One profile provides evidence for a velocity decrease as a function of r -1/2, instead of r -1, suggestive of surface friction effects. However, other profiles do not support this observation, or do not contain enough measurements to produce meaningful insights. Copyright 2011 by the American Geophysical Union. Source

Stecker F.W.,NASA | Scully S.T.,James Madison University
Astrophysical Journal Letters | Year: 2010

We derive a relation for the steepening of blazar γ-ray spectra between the multi-GeV Fermi energy range and the TeV energy range observed by atmospheric Cerenkov telescopes. The change in spectral index is produced by two effects: (1) an intrinsic steepening, independent of redshift, owing to the properties of emission and absorption in the source and (2) a redshift-dependent steepening produced by intergalactic pair production interactions of blazar γ-rays with low-energy photons of the "intergalactic background light" (IBL). Given this relation, with good enough data on the mean γ-ray spectral energy distribution of TeV-selected BL Lac objects, the redshift evolution of the IBL can, in principle, be determined independently of stellar evolution models. We apply our relation to the results of new Fermi observations of TeV-selected blazars. © 2010. The American Astronomical Society. All rights reserved. Source

Jethva H.,Hampton University | Torres O.,NASA
Atmospheric Chemistry and Physics | Year: 2011

We provide satellite-based evidence of the spectral dependence of absorption in biomass burning aerosols over South America using near-UV measurements made by the Ozone Monitoring Instrument (OMI) during 2005-2007. In the current near-UV OMI aerosol algorithm (OMAERUV), it is implicitly assumed that the only absorbing component in carbonaceous aerosols is black carbon whose imaginary component of the refractive index is wavelength independent. With this assumption, OMI-derived aerosol optical depth (AOD) is found to be significantly over-estimated compared to that of AERONET at several sites during intense biomass burning events (August-September). Other well-known sources of error affecting the near-UV method of aerosol retrieval do not explain the large observed AOD discrepancies between the satellite and the ground-based observations. A number of studies have revealed strong spectral dependence in carbonaceous aerosol absorption in the near-UV region suggesting the presence of organic carbon in biomass burning generated aerosols. A sensitivity analysis examining the importance of accounting for the presence of wavelength-dependent aerosol absorption in carbonaceous particles in satellite-based remote sensing was carried out in this work. The results convincingly show that the inclusion of spectrally-dependent aerosol absorption in the radiative transfer calculations leads to a more accurate characterization of the atmospheric load of carbonaceous aerosols. The use of a new set of aerosol models assuming wavelength-dependent aerosol absorption in the near-UV region (Absorption Angstrom Exponent λ-2.5 to -3.0) improved the OMAERUV retrieval results by significantly reducing the AOD bias observed when gray aerosols were assumed. In addition, the new retrieval of single-scattering albedo is in better agreement with those of AERONET within the uncertainties (ΔSSA Combining double low line ±0.03). The new colored carbonaceous aerosol model was also found to reproduce the ground-based AOD observations over the biomass burning region of central Africa and northern India. Together with demonstrating a significant improvement in the retrieval of aerosol properties from OMI, the present study highlights the greater sensitivity of the near-UV measurements to the varying spectral aerosol absorption. This capability can be explored further for the use in the identification of the black carbon and organics in the biomass burning aerosols. © 2011 Author(s). Source

Guyon O.,Japan National Astronomical Observatory | Guyon O.,University of Arizona | Martinache F.,Japan National Astronomical Observatory | Belikov R.,NASA | Soummer R.,US Space Telescope Science Institute
Astrophysical Journal, Supplement Series | Year: 2010

We describe a coronagraph approach where the performance of a Phase-Induced Amplitude Apodization (PIAA) coronagraph is improved by using a partially transmissive phase-shifting focal plane mask and a Lyot stop. This approach combines the low inner working angle offered by phase mask coronagraphy, the full throughput and uncompromized angular resolution of the PIAA approach, and the design flexibility of Apodized Pupil Lyot Coronagraph. A PIAA complex mask coronagraph (PIAACMC) is fully described by the focal plane mask size, or, equivalently, its complex transmission which ranges from 0 (opaque) to -1 (phase shifting). For all values of the transmission, the PIAACMC theoretically offers full on-axis extinction and 100% throughput at large angular separations. With a pure phase focal plane mask (complex transmission = -1), the PIAACMC offers 50% throughput at 0.64 λ/D while providing total extinction of an on-axis point source. This performance is very close to the "fundamental performance limit" of coronagraphy derived from first principles. For very high contrast level, imaging performance with PIAACMC is in practice limited by the angular size of the on-axis target (usually a star). We show that this fundamental limitation must be taken into account when choosing the optimal value of the focal plane mask size in the PIAACMC design. We show that the PIAACMC enables visible imaging of Jupiter-like planets at ≈1.2 λ/D from the host star, and can therefore offer almost three times more targets than a PIAA coronagraph optimized for this type of observation. We find that for visible imaging of Earthlike planets, the PIAACMC gain over a PIAA is probably much smaller, as coronagraphic performance is then strongly constrained by stellar angular size. For observations at "low" contrast (below ≈108), the PIAACMC offers significant performance enhancement over PIAA. This is especially relevant for ground-based high contrast imaging systems in the near-IR, where PIAACMC enables high contrast high efficiency imaging within 1 λ/D. Manufacturing tolerances for the focal plane mask are quantified for a few representative PIAACMC designs. © 2010. The American Astronomical Society. All rights reserved. Source

Duvall Jr. T.L.,NASA | Birch A.C.,NorthWest Research Associates, Inc.
Astrophysical Journal Letters | Year: 2010

Supergranules are observed at the solar photosphere as a cellular horizontal flow pattern with flow diverging from cell centers and converging on cell boundaries. Clark & Johnson calculated that mass conservation leads to an expected vertical flow of only 10ms-1, which has been difficult to observe. In the present work, Doppler images near the disk center from Michelson Doppler Imager are averaged about locations of cell centers to obtain the necessary signal-to-noise ratio to see the vertical flow. It is found that, for an average over 1100 cell centers, there is a 10ms-1 upflow at cell center and a 5ms-1 downflow at the cell boundaries, confirming the previous estimate. The rms vertical flow is 4ms-1, smaller than Giovanelli's upper limit of 10ms-1. © 2010. The American Astronomical Society. All rights reserved. Source

Lepping R.P.,NASA | Wu C.-C.,Navel Research Laboratory
Annales Geophysicae | Year: 2010

This study is motivated by the unusually low number of magnetic clouds (MCs) that are strictly identified within interplanetary coronal mass ejections (ICMEs), as observed at 1 AU; this is usually estimated to be around 30% or lower. But a looser definition of MCs may significantly increase this percentage. Another motivation is the unexpected shape of the occurrence distribution of the observers' "closest approach distances" (measured from a MC's axis, and called CA) which drops off somewhat rapidly as |CA| (in % of MC radius) approaches 100%, based on earlier studies. We suggest, for various geometrical and physical reasons, that the |CA|-distribution should be somewhere between a uniform one and the one actually observed, and therefore the 30% estimate should be higher. So we ask, When there is a failure to identify a MC within an ICME, is it occasionally due to a large |CA| passage, making MC identification more difficult, i.e., is it due to an event selection effect? In attempting to answer this question we examine WIND data to obtain an accurate distribution of the number of MCs vs. |CA| distance, whether the event is ICME-related or not, where initially a large number of cases (N=98) are considered. This gives a frequence distribution that is far from uniform, confirming earlier studies. This along with the fact that there are many ICME identification-parameters that do not depend on |CA| suggest that, indeed an MC event selection effect may explain at least part of the low ratio of (No. MCs)/(No. ICMEs). We also show that there is an acceptable geometrical and physical consistency in the relationships for both average " normalized" magnetic field intensity change and field direction change vs. |CA| within a MC, suggesting that our estimates of |CA|, BO (magnetic field intensity on the axis), and choice of a proper "cloud coordinate" system (all needed in the analysis) are acceptably accurate. Therefore, the MC fitting model (Lepping et al., 1990) is adequate, on average, for our analysis. However, this selection effect is not likely to completely answer our original question, on the unexpected ratio of MCs to ICMEs, so we must look for other factors, such as peculiarities of CME birth conditions. As a by-product of this analysis, we determine that the first order structural effects within a MC due to its interaction with the solar wind, plus the MC's usual expansion at 1 AU (i.e., the non-force free components of the MC's field) are, on average, weakly dependent on radial distance from the MC's axis; that is, in the outer reaches of a typical MC the non-force free effects show up, but even there they are rather weak. Finally, we show that it is not likely that a MC's size distribution statistically controls the occurrence distribution of the estimated |CA|s. © Author(s) 2010. Source

Wolff C.L.,NASA | Patrone P.N.,University of Maryland University College
Solar Physics | Year: 2010

We derive a perturbation inside a rotating star that occurs when the star is accelerated by orbiting bodies. If a fluid element has rotational and orbital components of angular momentum with respect to the inertially fixed point of a planetary system that are of opposite sign, then the element may have potential energy that could be released by a suitable flow. We demonstrate the energy with a very simple model in which two fluid elements of equal mass exchange positions, calling to mind a turbulent field or natural convection. The exchange releases potential energy that, with a minor exception, is available only in the hemisphere facing the barycenter of the planetary system. We calculate its strength and spatial distribution for the strongest case ("vertical") and for weaker horizontal cases whose motions are all perpendicular to gravity. The vertical cases can raise the kinetic energy of a few well positioned convecting elements in the Sun's envelope by a factor ≤7. This is the first physical mechanism by which planets can have a nontrivial effect on internal solar motions. Occasional small mass exchanges near the solar center and in a recently proposed mixed shell centered at 0.16Rs would carry fresh fuel to deeper levels. This would cause stars like the Sun with appropriate planetary systems to burn somewhat more brightly and have shorter lifetimes than identical stars without planets. The helioseismic sound speed and the long record of sunspot activity offer several bits of evidence that the effect may have been active in the Sun's core, its envelope, and in some vertically stable layers. Additional proof will require direct evidence from helioseismology or from transient waves on the solar surface. © 2010 US Government. Source

Burlaga L.F.,NASA | Ness N.F.,Catholic University of America
Astrophysical Journal Letters | Year: 2014

Voyager 1 (V1) was beyond the heliopause between 2013.00 and 2014.41, where it was making in situ observations of the interstellar magnetic field (ISMF). The average azimuthal angle and elevation angle of the magnetic field B were 〈λ〉 = 292small letter o with dot below5 ± 1small letter o with dot below4 and 〈δ〉 = 22small letter o with dot below1 ± 1small letter o with dot below2, respectively. The angles λ and δ varied linearly at (1small letter o with dot below4 ± 0small letter o with dot below1) yr-1and (-1small letter o with dot below1 ± 0small letter o with dot below1) yr-1, respectively, suggesting that V1 was measuring the draped ISMF around the heliopause. The distributions of hourly averages of λ and δ were Gaussian distributions, with most probable values 292small letter o with dot below5 and 22small letter o with dot below1, and standard deviations (SDs) 1small letter o with dot below3 and 1small letter o with dot below1, respectively. The small SD indicates little or no turbulence transverse to B. An abrupt decrease in B from 0.50 nT on 2013/129.9 to 0.46 nT on 2013/130.6 was observed, possibly associated with a weak reverse shock or magnetoacoustic pressure wave following a burst of electron plasma oscillations. Between 2013/130.6 and 2013/365.3, 〈B〉 = 0.464 ± 0.009 nT, 〈λ〉 = 292small letter o with dot below6 ± 0small letter o with dot below8, and 〈δ〉 = 22small letter o with dot below1 ± 1small letter o with dot below1. The corresponding distribution of hourly averages of B was Gaussian with the most probable value 0.464 nT and σ = 0.009 nT. Since the uncertainty σ corresponds to the instrument and digitization noise, these observations provided an upper limit to the turbulence in the ISMF. The distributions of the hourly increments of B were Gaussian distributions with σ = 0.05 nT, 0small letter o with dot below4, and 0small letter o with dot below4, respectively, indicating that the V1 did not detect evidence of "intermittent bursts" of interstellar turbulence. © 2014. The American Astronomical Society. All rights reserved. Source

Strohmayer T.,NASA | Mahmoodifar S.,University of Maryland University College
Astrophysical Journal Letters | Year: 2014

Neutron stars are among the most compact objects in the universe and provide a unique laboratory for the study of cold ultra-dense matter. While asteroseismology can provide a powerful probe of the interiors of stars, for example, helioseismology has provided unprecedented insights about the interior of the Sun, comparable capabilities for neutron star seismology have not yet been achieved. Here, we report the discovery of a coherent X-ray modulation from the neutron star 4U 1636-536 during the 2001 February 22 thermonuclear superburst seen with NASA's Rossi X-Ray Timing Explorer (RXTE) that is very likely produced by a global oscillation mode. The observed frequency is 835.6440 ± 0.0002 Hz (1.43546 times the stellar spin frequency of 582.14323 Hz) and the modulation is well described by a sinusoid (A + Bsin (φ-φ0)) with a fractional half-amplitude of B/A = 0.19 ± 0.04% (4-15 keV). The observed frequency is consistent with the expected inertial frame frequency of a rotationally modified surface g-mode, an interfacial mode in the ocean-crust interface, or perhaps an r-mode. Observing an inertial frame frequency - as opposed to a co-rotating frame frequency - appears consistent with the superburst's thermal emission arising from the entire surface of the neutron star, and the mode may become visible by perturbing the local surface temperature. We briefly discuss the implications of the mode detection for the neutron star's projected velocity and mass. Our results provide further strong evidence that global oscillation modes can produce observable modulations in the X-ray flux from neutron stars. © 2014. The American Astronomical Society. All rights reserved.. Source

Lau K.-M.,NASA | Wu H.-T.,Science Systems And Applications Inc.
Journal of Geophysical Research: Atmospheres | Year: 2011

This study investigates the climatological and changing characteristics of tropical rain and cloud systems in relation to sea surface temperature (SST) changes using Tropical Rainfall Measuring Mission (TRMM) data (1998-2009). Rainfall and cloud characteristics are determined from probability distribution functions (pdf), derived from daily TRMM Microwave Imager (TMI) and Precipitation Radar (PR) surface rain, Visible and Infrared Scanner (VIRS) brightness temperature (Tb), and PR echo top height (HET). Results show that the top 10% heavy rain is associated with cold cloud tops (Tb < 220 K) and elevated echo top heights (HET > 6 km), associated with ice phase rain in the Intertropical Convergence Zone and monsoon regions. The bottom 5% light rain occurs most frequently in the subtropics and also in the warm pool regions with low cloud top (Tb > 273 K) and HET ∼ 1-4 km. Intermediate rain (25th to 75th percentile) is contributed by a wide range of middle clouds and mixed-phase rain centered at Tb ∼ 230-260 K and HET ∼ 4-6 km within the warm pool. The relationships between rainfall and SST depend strongly on rain types. We find that a warmer tropical ocean favors a large increase in annual heavy rain accumulation, a mild reduction in light to moderate rain, and a slight increase in extremely light rain. The annual accumulation of extreme heavy rain increases approximately 80%-90% for every degree rise in SST, much higher than that expected from the Clausius-Clapeyron equation for global water balance. This is possibly because heavy rain is only a component of the tropical water cycle and is strongly associated with ice phase processes and convective dynamics feedback. Copyright 2011 by the American Geophysical Union. Source

Taylor P.C.,NASA | Ellingson R.G.,Florida State University | Cai M.,Florida State University
Journal of Climate | Year: 2011

This study investigates the annual cycle of radiative contributions to global climate feedbacks. A partial radiative perturbation (PRP) technique is used to diagnose monthly radiative perturbations at the top of atmosphere (TOA) due to CO 2 forcing; surface temperature response; and water vapor, cloud, lapse rate, and surface albedo feedbacks using NCAR Community Climate System Model, version 3 (CCSM3) output from a Special Report on Emissions Scenarios (SRES) A1B emissions-scenario-forced climate simulation. The seasonal global mean longwave TOA radiative feedback was found to be minimal. However, the global mean shortwave (SW) TOA cloud and surface albedo radiative perturbations exhibit large seasonality. The largest contributions to the negative SW cloud feedback occur during summer in each hemisphere, marking the largest differences with previous results. Results suggest that intermodel spread in climate sensitivity may occur, partially from cloud and surface albedo feedback seasonality differences. Further, links between the climate feedback and surface temperature response seasonality are investigated, showing a strong relationship between the seasonal climate feedback distribution and the seasonal surface temperature response. © 2011 American Meteorological Society. Source

Furlanetto S.R.,University of California at Los Angeles | Furlanetto S.R.,NASA | Stoever S.J.,Cornell University
Monthly Notices of the Royal Astronomical Society | Year: 2010

We examine the fate of fast electrons (with energies E > 10 eV) in a thermal gas of primordial composition. To follow their interactions with the background gas, we construct a Monte Carlo model that includes: (1) electron-electron scattering (which transforms the electron kinetic energy into heat), (2) collisional ionization of hydrogen and helium (which produces secondary electrons that themselves scatter through the medium) and (3) collisional excitation (which produces secondary photons, whose fates we also follow approximately). For the last process, we explicitly include all transitions to upper levels n ≤ 4, together with a well-motivated extrapolation to higher levels. In all cases, we use recent calculated cross-sections at E < 1 keV and the Bethe approximation to extrapolate to higher energies. We compute the fractions of energy deposited as heat, ionization (tracking H i and the helium species separately) and excitation (tracking H i Lyα separately) under a broad range of conditions appropriate to the intergalactic medium. The energy deposition fractions depend on both the background ionized fraction and the electron energy but are nearly independent of the background density. We find good agreement with some, but not all, previous calculations at high energies. Electronic tables of our results are available. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Yonekura E.,Columbia University | Hall T.M.,NASA
Journal of Applied Meteorology and Climatology | Year: 2011

A new statistical model for western North Pacific Ocean tropical cyclone genesis and tracks is developed and applied to estimate regionally resolved tropical cyclone landfall rates along the coasts of the Asian mainland, Japan, and the Philippines. The model is constructed on International Best Track Archive for Climate Stewardship (IBTrACS) 1945-2007 historical data for the western North Pacific. The model is evaluated in several ways, including comparing the stochastic spread in simulated landfall rates with historic landfall rates. Although certain biases have been detected, overall the model performs well on the diagnostic tests, for example, reproducing well the geographic distribution of landfall rates. Western North Pacific cyclogenesis is influenced by El Niño-Southern Oscillation (ENSO). This dependence is incorporated in the model's genesis component to project the ENSO-genesis dependence onto landfall rates. There is a pronounced shift southeastward in cyclogenesis and a small but significant reduction in basinwide annual counts with increasing ENSO index value. On almost all regions of coast, landfall rates are significantly higher in a negative ENSO state (La Niña). © 2011 American Meteorological Society. Source

Hodges R.R.,University of Colorado at Boulder | Hodges R.R.,NASA
Geophysical Research Letters | Year: 2011

The recent discovery, by instruments on the IBEX and Chandrayaan-1 spacecraft, of significant fluxes of energetic hydrogen atoms escaping from the moon suggests that there are flaws in the common wisdom regarding the interaction of solar wind with lunar regolith that have prevailed in attempts to explain the failure of the Apollo 17 far-ultraviolet spectrometer to detect neutral H in the lunar exosphere. A new theory for the interaction of solar wind with the lunar regolith surface is tested by comparing simulated spectra of reflected energetic neutral hydrogen and protons with analogous neutral spectra from Chandrayaan-1 and proton data from the Kaguya mission. Overall, the theory indicates that roughly 1% of solar wind protons incident on the lunar regolith surface exit as energetic protons, and about 98.5% exit as neutral H with super-escape speeds. The remaining 0.5%, which exit as neutral H atoms with sub-escape speeds, form a tenuous exosphere that is compatible with the levels of Lyman- allowed by Apollo 17 observations. © 2011 by the American Geophysical Union. Source

Schmerr N.C.,NASA | Kelly B.M.,University of Calgary | Thorne M.S.,University of Utah
Geophysical Research Letters | Year: 2013

Intermittent seismic discontinuities near 250-300 km depth beneath South America and the Pacific basin are detected with high-resolution seismic array methods that use SS and PP precursors recorded at the High Lava Plains Seismic Experiment and the EarthScope Transportable Array. The transformation of coesite to stishovite in an eclogite-rich mantle composition produces a seismic discontinuity near 300 km depth; lateral changes in basalt fraction of the upper mantle will thus produce an intermittent seismic discontinuity. The sensitivity of the precursors to intermittent seismic structure is addressed using an axisymmetric finite difference model of wave propagation in the mantle. These numerical experiments find that the precursors are sensitive to structures ≥500 km in lateral extent and that the observations of this discontinuity are plausibly tied to lateral variations in basaltic composition of the upper mantle related to dynamics, such as plumes and subduction. © 2013. American Geophysical Union. All Rights Reserved. Source

Cook B.I.,NASA | Cook B.I.,Lamont Doherty Earth Observatory | Seager R.,Lamont Doherty Earth Observatory
Journal of Geophysical Research: Atmospheres | Year: 2013

We analyze the response of the North American Monsoon (NAM) to increased greenhouse gas (GHG) forcing (emissions scenario RCP 8.5) using new simulations available through the Coupled Model Intercomparison Project version 5 (CMIP5). Changes in total monsoon season rainfall with GHG warming are small and insignificant. The models do, however, show significant declines in early monsoon season precipitation (June-July) and increases in late monsoon season (September-October) precipitation, indicating a shift in seasonality toward delayed onset and withdrawal of the monsoon. Early in the monsoon season, tropospheric warming increases vertical stability, reinforced by reductions in available surface moisture, inhibiting precipitation and delaying the onset of the monsoon. By the end of the monsoon season, moisture convergence is sufficient to overcome the warming induced stability increases, and precipitation is enhanced. Even with no change in total NAM rainfall, shifts in the seasonal distribution of precipitation within the NAM region are still likely to have significant societal and ecological consequences, reinforcing the need to not only understand the magnitude, but also the timing, of future precipitation changes. Key PointsWarming suppresses early monsoon rainfall and enhances late monsoon rainfallEarly declines are caused by increased stability due to upper level warmingLater increases occur once moisture convergence can overcome increased stability © 2013. American Geophysical Union. All Rights Reserved. Source

Rightmire-Upton L.,University of Alabama in Huntsville | Hathaway D.H.,NASA | Kosak K.,Florida Institute of Technology
Astrophysical Journal Letters | Year: 2012

The meridional circulation at high latitudes is crucial to the buildup and reversal of the Sun's polar magnetic fields. Here, we characterize the axisymmetric flows by applying a magnetic feature cross-correlation procedure to high-resolution magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We focus on Carrington rotations 2096-2107 (2010 April to 2011 March) - the overlap interval between HMI and the Michelson Doppler Imager (MDI). HMI magnetograms averaged over 720 s are first mapped into heliographic coordinates. Strips from these maps are then cross-correlated to determine the distances in latitude and longitude that the magnetic element pattern has moved, thus providing meridional flow and differential rotation velocities for each rotation of the Sun. Flow velocities were averaged for the overlap interval and compared to results obtained from MDI data. This comparison indicates that these HMI images are rotated counterclockwise by 0075 with respect to the Sun's rotation axis. The profiles indicate that HMI data can be used to reliably measure these axisymmetric flow velocities to at least within 5° of the poles. Unlike the noisier MDI measurements, no evidence of a meridional flow counter-cell is seen in either hemisphere with the HMI measurements: poleward flow continues all the way to the poles. Slight north-south asymmetries are observed in the meridional flow. These asymmetries should contribute to the observed asymmetries in the polar fields and the timing of their reversals. © 2012. The American Astronomical Society. All rights reserved. Source

Harrison T.E.,New Mexico State University | Hamilton R.T.,NASA
Astronomical Journal | Year: 2015

We use a modified version of MOOG to generate large grids of synthetic spectra in an attempt to derive quantitative abundances for three CVs (GK Per, RU Peg, and SS Cyg) by comparing the models to moderate resolution (R ∼ 25,000) K-band spectra obtained with NIRSPEC on Keck. For each of the three systems we find solar, or slightly sub-solar values for [Fe/H], but significant deficits of carbon: for SS Cyg we find [C/Fe] = -0.50, for RU Peg [C/Fe] = -0.75, and for GK Per [C/Fe] = -1.00. We show that it is possible to use lower resolution (R ∼ 2000) spectra to quantify carbon deficits. We examine realistic veiling scenarios and find that emission from H i or CO cannot reproduce the observations. © 2015. The American Astronomical Society. All rights reserved. Source

The excessive precipitation over steep and high mountains (EPSM) in GCMs and mesoscale models is due to a lack of parameterization of the thermal effects of subgrid-scale topographic variation. These thermal effects drive subgrid-scale heated-slope-induced vertical circulations (SHVC). SHVC provide a ventilation effect of removing heat from the boundary layer of resolvable-scale mountain slopes and depositing it higher up. The lack of SHVC parameterization is the cause of EPSM. The author has previously proposed a method of parameterizing SHVC, here termed SHVC.1. Although this has been successful in avoiding EPSM, the drawback is that it suppresses convective-type precipitation in the regions where it is applied. In this article, the author proposes a new method of parameterizing SHVC, here termed SHVC.2. In SHVC.2, the potential temperature and mixing ratio of the boundary layer are changed when used as input to the cumulus parameterization scheme over mountainous regions. This allows the cumulus parameterization to assume the additional function of SHVC parameterization. SHVC.2 has been tested in NASA Goddard's GEOS-5 GCM. It achieves the primary goal of avoiding EPSM while also avoiding the suppression of convective-type precipitation in the regions where it is applied. © 2015 American Meteorological Society. Source

Cohen O.,Harvard - Smithsonian Center for Astrophysics | Glocer A.,NASA
Astrophysical Journal Letters | Year: 2012

Atmospheric mass loss from Hot Jupiters can be large due to the close proximity of these planets to their host star and the strong radiation the planetary atmosphere receives. On Earth, a major contribution to the acceleration of atmospheric ions comes from the vertical separation of ions and electrons, and the generation of the ambipolar electric field. This process, known as the "polar wind," is responsible for the transport of ionospheric constituents to Earth's magnetosphere, where they are well observed. The polar wind can also be enhanced by a relatively small fraction of super-thermal electrons (photoelectrons) generated by photoionization. We formulate a simplified calculation of the effect of the ambipolar electric field and the photoelectrons on the ion scale height in a generalized manner. We find that the ion scale height can be increased by a factor of 2-15 due to the polar wind effects. We also estimate a lower limit of an order of magnitude increase of the ion density and the atmospheric mass-loss rate when polar wind effects are included. © 2012. The American Astronomical Society. All rights reserved.. Source

Vaillancourt J.E.,NASA | Matthews B.C.,National Research Council Canada
Astrophysical Journal, Supplement Series | Year: 2012

The Hertz and SCUBA polarimeters, working at 350 μm and 850 μm, respectively, have measured the polarized emission in scores of Galactic clouds. Of the clouds in each data set, 17 were mapped by both instruments with good polarization signal-to-noise ratios. We present maps of each of these 17 clouds comparing the dual-wavelength polarization amplitudes and position angles at the same spatial locations. In total number of clouds compared, this is a four-fold increase over previous work. Across the entire data set real position angle differences are seen between wavelengths. While the distribution of φ(850)-φ(350) is centered near zero (near-equal angles), 64% of data points with high polarization signal-to-noise (P ≥ 3σ p) have |φ(850)-φ(350)| > 10°. Of those data with small changes in position angle (≤10°) the median ratio of the polarization amplitudes is P(850)/P(350) = 1.7 ± 0.6. This value is consistent with previous work performed on smaller samples and models that require mixtures of different grain properties and polarization efficiencies. Along with the polarization data we have also compiled the intensity data at both wavelengths; we find a trend of decreasing polarization with increasing 850-to-350 μm intensity ratio. All the polarization and intensity data presented here (1699 points in total) are available in electronic format. © 2012. The American Astronomical Society. All rights reserved. Source

Altamirano D.,University of Amsterdam | Strohmayer T.,NASA
Astrophysical Journal Letters | Year: 2012

We report the discovery of quasi-periodic oscillations (QPOs) at ∼ 11 mHz in two RXTE and one Chandra observations of the black hole candidate H1743-322. The QPO is observed only at the beginning of the 2010 and 2011 outbursts at similar hard color and intensity, suggestive of an accretion state dependence for the QPO. Although its frequency appears to be correlated with X-ray intensity on timescales of a day, in successive outbursts eight months apart, we measure a QPO frequency that differs by less than ≈2.2 mHz while the intensity had changed significantly. We show that this ∼11 mHz QPO is different from the so-called Type C QPOs seen in black holes and that the mechanisms that produce the two flavors of variability are most probably independent. After comparing this QPO with other variability phenomena seen in accreting black holes and neutron stars, we conclude that it best resembles the so-called 1Hz QPOs seen in dipping neutron star systems, although having a significantly lower (1-2 orders of magnitude) frequency. If confirmed, H1743-322 is the first black hole showing this type of variability. Given the unusual characteristics and the hard-state dependence of the ∼11 mHz QPO, we also speculate whether these oscillations could instead be related to the radio jets observed in H1743-322. A systematic search for this type of low-frequency QPOs in similar systems is needed to test this speculation. In any case, it remains unexplained why these QPOs have only been seen in the last two outbursts of H1743-322. © 2012. The American Astronomical Society. All rights reserved.. Source

Stacy A.,NASA | Bromm V.,University of Texas at Austin
Monthly Notices of the Royal Astronomical Society | Year: 2013

We perform a cosmological simulation in order to model the growth and evolution of Population III (Pop III) stellar systems in a range of host minihalo environments. A Pop III multiple system forms in each of the 10 minihaloes, and the overall mass function is top-heavy compared to the currently observed initial mass function in the Milky Way. Using a sink particle to represent each growing protostar, we examine the binary characteristics of the multiple systems, resolving orbits on scales as small as 20 au. We find a binary fraction of ~35 per cent, with semi-major axes as large as 3000 au. The distribution of orbital periods is slightly peaked at ≲ 900 yr, while the distribution of mass ratios is relatively flat. Of all sink particles formed within the 10 minihaloes, ~50 per cent are lost to mergers with larger sinks, and ~50 per cent of the remaining sinks are ejected from their star-forming discs. The large binary fraction may have important implications for Pop III evolution and nucleosynthesis, as well as the final fate of the first stars. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Pfaff Jr. R.F.,NASA
Space Science Reviews | Year: 2012

An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth's plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances. © 2012 US Government. Source

Shaposhnikov N.,University of Maryland University College | Shaposhnikov N.,NASA
Astrophysical Journal Letters | Year: 2012

Observations of quasi-periodic oscillations (QPOs) in X-ray binaries hold a key to understanding many aspects of these enigmatic systems. Complex appearance of the Fourier phase lags related to QPOs is one of the most puzzling observational effects in accreting black holes (BHs). In this Letter we show that QPO properties, including phase lags, can be explained in a framework of a simple scenario, where the oscillating media provide feedback on the emerging spectrum. We demonstrate that the QPO waveform is presented by the product of a perturbation and time-delayed response factors, where the response is energy dependent. The essential property of this effect is its nonlinear and multiplicative nature. Our multiplicative reverberation model successfully describes the QPO components in energy-dependent power spectra as well as the appearance of the phase lags between signals in different energy bands. We apply our model to QPOs observed by the Rossi X-Ray Timing Explorer in BH candidate XTE J1550-564. We briefly discuss the implications of the observed energy dependence of the QPO reverberation times and amplitudes on the nature of the power-law spectral component and its variability. © 2012. The American Astronomical Society. All rights reserved.. Source

Del Genio A.D.,NASA | Barbara J.M.,Sigma Space
Icarus | Year: 2012

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ∼10ms -1 weaker than cloud level winds in the cores of eastward jets and ∼5ms -1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2×10 -5m 2s -3) and smaller in westward jet regions (1.6×10 -5m 2s -3) than the global mean value (4.1×10 -5m 2s -3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level. © 2012 . Source

Barahona D.,NASA
Atmospheric Chemistry and Physics | Year: 2015

Cirrus clouds play a key role in the radiative and hydrological balance of the upper troposphere. Their correct representation in atmospheric models requires an understanding of the microscopic processes leading to ice nucleation. A key parameter in the theoretical description of ice nucleation is the activation energy, which controls the flux of water molecules from the bulk of the liquid to the solid during the early stages of ice formation. In most studies it is estimated by direct association with the bulk properties of water, typically viscosity and self-diffusivity. As the environment in the ice-liquid interface may differ from that of the bulk, this approach may introduce bias in calculated nucleation rates. In this work a theoretical model is proposed to describe the transfer of water molecules across the ice-liquid interface. Within this framework the activation energy naturally emerges from the combination of the energy required to break hydrogen bonds in the liquid, i.e., the bulk diffusion process, and the work dissipated from the molecular rearrangement of water molecules within the ice-liquid interface. The new expression is introduced into a generalized form of classical nucleation theory. Even though no nucleation rate measurements are used to fit any of the parameters of the theory the predicted nucleation rate is in good agreement with experimental results, even at temperature as low as 190 K, where it tends to be underestimated by most models. It is shown that the activation energy has a strong dependency on temperature and a weak dependency on water activity. Such dependencies are masked by thermodynamic effects at temperatures typical of homogeneous freezing of cloud droplets; however, they may affect the formation of ice in haze aerosol particles. The new model provides an independent estimation of the activation energy and the homogeneous ice nucleation rate, and it may help to improve the interpretation of experimental results and the development of parameterizations for cloud formation. © 2015 Author(s). Source

Wolkovich E.M.,University of British Columbia | Cook B.I.,NASA | Cook B.I.,Lamont Doherty Earth Observatory | Davies T.J.,McGill University
New Phytologist | Year: 2014

Climate change has brought renewed interest in the study of plant phenology - the timing of life history events. Data on shifting phenologies with warming have accumulated rapidly, yet research has been comparatively slow to explain the diversity of phenological responses observed across latitudes, growing seasons and species. Here, we outline recent efforts to synthesize perspectives on plant phenology across the fields of ecology, climate science and evolution. We highlight three major axes that vary among these disciplines: relative focus on abiotic versus biotic drivers of phenology, on plastic versus genetic drivers of intraspecific variation, and on cross-species versus autecological approaches. Recent interdisciplinary efforts, building on data covering diverse species and climate space, have found a greater role of temperature in controlling phenology at higher latitudes and for early-flowering species in temperate systems. These efforts have also made progress in understanding the tremendous diversity of responses across species by incorporating evolutionary relatedness, and linking phenological flexibility to invasions and plant performance. Future research with a focus on data collection in areas outside the temperate mid-latitudes and across species' ranges, alongside better integration of how risk and investment shape plant phenology, offers promise for further progress. © 2013 New Phytologist Trust. Source

Smialek J.L.,NASA
Corrosion Science | Year: 2015

Ti3AlC2, Ti2AlC, and Cr2AlC are oxidation resistant MAX phase compounds distinguished by the formation of protective Al2O3 scales with well controlled kinetics. A modified Wagner treatment was used to calculate interfacial grain boundary diffusivity from scale growth rates and corresponding interfacial grain size, based on the pressure dependence of oxygen vacancies and diffusivity. MAX phase data from the literature yielded grain boundary diffusivity nearly coincident with that for Zr-doped FeCrAl (and many other FeCrAl alloys), suggesting similar oxidation mechanisms. The consolidated body of diffusivity data was consistent with an activation energy of 375±25kJ/mol. © 2014. Source

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam specimen is used to measure fracture toughness, GIc, and strain energy release rate, GImax, for delamination onset and growth in laminated composites under Mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy double-cantilevered beam specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional Fiber-Reinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, GIR, for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic GImax level equal to 90% of the fracture toughness, GIc, to determine the delamination growth rate. Two different data reduction methods, a two-point and a seven-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the non-normalized results. Paris Law exponents were found to decrease by 5.7 to 47.6% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at three lower cyclic GImax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material. © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav. Source

Lau W.K.-M.,NASA | Wu H.-T.,Science Systems And Applications Inc. | Kim K.-M.,Morgan State University
Geophysical Research Letters | Year: 2013

In this study, we find from analyses of projections of 14 CMIP5 models a robust, canonical global response in rainfall characteristics to a warming climate. Under a scenario of 1% increase per year of CO2 emission, the model ensemble projects globally more heavy precipitation (+7 - 2.4%K -1), less moderate precipitation (-2.5 - 0.6%K-1), more light precipitation (+1.8 - 1.3%K-1), and increased length of dry (no-rain) periods (+4.7 - 2.1%K-1). Regionally, a majority of the models project a consistent response with more heavy precipitation over climatologically wet regions of the deep tropics, especially the equatorial Pacific Ocean and the Asian monsoon regions, and more dry periods over the land areas of the subtropics and the tropical marginal convective zones. Our results suggest that increased CO2 emissions induce a global adjustment in circulation and moisture availability manifested in basic changes in global precipitation characteristics, including increasing risks of severe floods and droughts in preferred geographic locations worldwide. Key Points A canonical rainfall response is found in CMIP-5 models increased floods and droughts under global warming are connected Changing rainfall types are more sensitive than total rainfall ©2013. American Geophysical Union. All Rights Reserved. Source

Mishchenko M.I.,NASA
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2011

Measurements with directional radiometers and calculations based on the radiative transfer equation (RTE) have been at the very heart of weather and climate modeling and terrestrial remote sensing. The quantification of the energy budget of the Earth's climate system requires exquisite measurements and computations of the incoming and outgoing electromagnetic energy, while global characterization of climate system's components relies heavily on theoretical inversions of observational data obtained with various passive and active instruments. The same basic problems involving electromagnetic energy transport and its use for diagnostic and characterization purposes are encountered in numerous other areas of science, biomedicine, and engineering. Yet both the discipline of directional radiometry and the radiative transfer theory (RTT) have traditionally been based on phenomenological concepts many of which turn out to be profound misconceptions. Contrary to the widespread belief, a collimated radiometer does not, in general, measure the flow of electromagnetic energy along its optical axis, while the specific intensity does not quantify the amount of electromagnetic energy transported in a given direction.The recently developed microphysical approach to radiative transfer and directional radiometry is explicitly based on the Maxwell equations and clarifies the physical nature of measurements with collimated radiometers and the actual content of the RTE. It reveals that the specific intensity has no fundamental physical meaning besides being a mathematical solution of the RTE, while the RTE itself is nothing more than an intermediate auxiliary equation. Only under special circumstances detailed in this review can the solution of the RTE be used to compute the time-averaged local Poynting vector as well as be measured by a collimated radiometer. These firmly established facts make the combination of the RTE and a collimated radiometer useful in a well-defined range of applications. However, outside the domain of validity of the RTT the practical usefulness of measurements with collimated radiometers remains uncertain, while the theoretical modeling of these measurements and the solution of the energy-budget problem require a more sophisticated approach than solving the RTE. © 2011. Source

Nemeth N.N.,NASA
Journal of Composite Materials | Year: 2014

The Batdorf "unit-sphere" methodology has been extended to predict the multiaxial stochastic strength response of anisotropic (specifically transversely isotropic) brittle materials, including polymer matrix composites, by considering (1) nonrandom orientation of intrinsic flaws and (2) critical strength or fracture toughness changing with flaw orientation relative to the material microstructure. The equations developed to characterize these properties are general and can model tightly defined or more diffuse material anisotropy textures describing flaw populations. In this paper, results from finite element analysis of a fiber-reinforced matrix unit cell were used with the unit-sphere model to predict the biaxial strength response of a unidirectional polymer matrix composite previously reported from the World-Wide Failure Exercise. Findings regarding stress-state interactions, thermal residual stresses, and failure modes are also provided. The unit-sphere methodology is an attempt to provide an improved mechanistic basis to the problem of predicting strength response of an anisotropic and composite material under multiaxial loading as compared to polynomial interaction equation formulations. The methodology includes consideration of strength scatter to predict material probability of failure, shear sensitivity of flaws, and accounting for multiple failure modes regarding overall failure response. © 2013 The Author(s). Source

Bell M.S.,NASA
Meteoritics and Planetary Science | Year: 2016

A study of pure, single crystal calcite shocked to pressures from 9.0 to 60.8 GPa was conducted to address contradictory data for carbonate shock behavior. The recovered materials were analyzed optically and by transmission electron microscopy (TEM), as well as by thermogravimetry (TGA), X-ray diffraction (XRD), and Raman-spectroscopy. In thin section, progressive comminution of calcite is observed although grains remain birefringent to at least 60.8 GPa. TGA analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure (R = 0.77) and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range. XRD patterns of shocked Iceland spar samples produce peaks that are qualitatively and quantitatively less intense, more diffuse, and shift to lower o2θ. However, the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60.8 GPa. Raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening. TEM micrographs display progressively diminishing crystallite domain sizes. Selected area electron diffraction (SAED) patterns reveal no signatures of amorphous material. These data show that essentially intact calcite is recovered at shock pressures up to 60.8 GPa with only slight mass loss (~7%). This work suggests that the amount of CO2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been (substantially) overestimated. © 2016 The Meteoritical Society. Source

Jacobson N.S.,NASA
Journal of the American Ceramic Society | Year: 2014

Rare-earth silicate compounds, such as those in the Y2O 3-SiO2 system, are promising candidates for coatings of SiC-based ceramics and ceramic matrix composites in combustion environments. The predicted lower activity of silica in these silicates will lead to less reactivity with the water vapor combustion products. A procedure for measuring silica activities in this system is discussed. Knudsen effusion mass spectrometry is used and the measured vapor pressure of SiO(g) is correlated to activity. Due to the low vapor pressure of SiO(g) in the temperature range of interest, a reducing agent is utilized to boost this vapor pressure without altering the solid-state composition. In addition, corrections are made for nonequilibrium vaporization. The measured silica activities are: Y 2O3 + Y2O3·SiO2 two phase field: log[a(SiO2)] = -5200.26(1/T) + 0.0567 (1532 < T(K) < 1670) Y2O3·SiO2 + Y 2O3·2SiO2 two phase field: log[a(SiO2)] = 4.2252(1/T)-0.5531 (1628 < T(K) < 1747) It is shown how these results can be used to predict reduced volatilization rates of the coatings. © Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Source

Morris P.J.,Pennsylvania State University | Zaman K.B.M.Q.,NASA
Journal of Sound and Vibration | Year: 2010

This paper describes an experimental investigation of the statistical properties of turbulent velocity fluctuations in an axisymmetric jet. The focus is on those properties that are relevant to the prediction of noise. Measurements are performed using two single hot-wire anemometers as well as a two-component anemometer. Two-point cross correlations of the axial velocity fluctuations and of the fluctuations in the square of the axial velocity fluctuations are presented. Several reference locations in the jet are used including points on the jet lip and centerline. The scales of the turbulence and the convection velocity are determined, both in an overall sense as well as a function of frequency. The relationship between the second and fourth order correlations is developed and compared with the experimental data. The implications of the use of dimensional as well as non-dimensional correlations are considered. Finally, a comparison is made between the length scales deduced from the flow measurements and a RANS CFD calculation. © 2009 Elsevier Ltd. Source

Morelli E.A.,NASA
Journal of Aircraft | Year: 2012

A technique for estimating aerodynamic parameters in real time using flight data without air flow angle measurements is described and demonstrated. The method is applied to simulated F-16 data and to flight data from a subscale jet transport aircraft. Modeling results obtained with the new approach using flight data without air flow angle measurements are compared with modeling results computed conventionally using flight data that includes air flow angle measurements. Comparisons demonstrate that the new technique can provide accurate aerodynamic modeling results without air flow angle measurements, which are often difficult and expensive to obtain. Implications for efficient flight testing and flight safety are discussed. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Source

Remsberg E.,NASA | Lingenfelser G.,Enterprise Parkway
Atmospheric Chemistry and Physics | Year: 2010

Stratospheric Aerosol and Gas Experiment (SAGE II) Version 6.2 ozone profiles are analyzed for their decadal-scale responses in the middle and upper stratosphere from September 1991 to August 2005. The profile data are averaged within twelve, 20A°-wide latitude bins from 55A° S to 55A° N and at twelve altitudes from 27.5 to 55.0 km. The separate, 14-yr data time series are analyzed using multiple linear regression (MLR) models that include seasonal, 28 and 21-month, 11-yr sinusoid, and linear trend terms. Proxies are not used for the 28-mo (QBO-like), 11-yr solar uv-flux, or reactive chlorine terms. Instead, the present analysis focuses on the periodic 11-yr terms to see whether they are in-phase with that of a direct, uv-flux forcing or are dominated by some other decadal-scale influence. It is shown that they are in-phase over most of the latitude/altitude domain and that they have max minus min variations between 25A° S and 25A° N that peak near 4% between 30 and 40 km. Model simulations of the direct effects of uv-flux forcings agree with this finding. The shape of the 11-yr ozone response profile from SAGE II also agrees with that diagnosed for the stratosphere over the same time period from the HALOE data. Ozone in the middle stratosphere of the northern subtropics is perturbed during 1991-1992 following the eruption of Pinatubo, and there are pronounced decadal-scale variations in the ozone of the upper stratosphere for the northern middle latitudes presumably due to dynamical forcings. The 11-yr ozone responses of the southern hemisphere appear to be free of those extra influences. The associated linear trend terms from the SAGE II analyses are slightly negative (-2 to -4%/decade) between 35 and 45 km and nearly constant across latitude. This finding is consistent with the fact that ozone is estimated to have decreased by no more than 1.5% due to the increasing chlorine from mid-1992 to about 2000 but with little change thereafter. It is concluded that a satellite, solar occultation measurement provides both the signal sensitivity and the vertical resolution to record the stratospheric ozone response to the forcing from the solar uv-flux, as well as those due to any other long-term changes. © 2010 Author(s). Source

Catania G.A.,University of Texas at Austin | Neumann T.A.,NASA
Geophysical Research Letters | Year: 2010

Surface melting on the Greenland Ice Sheet is common up to ∼1400 m elevation and, in extreme melt years, even higher. Water produced on the ice sheet surface collects in lakes and drains over the ice sheet surface via supraglacial streams and through the ice sheet via moulins. Water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally. Here we use ice-penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred. We find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years. We find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years. Our observations indicate that once established, these persistent moulins might be capable of establishing well-connected meltwater drainage pathways. © Copyright 2010 by the American Geophysical Union. Source

Del Genio A.D.,NASA | Wu J.,Columbia University
Journal of Climate | Year: 2010

In continental convective environments, general circulation models typically produce a diurnal cycle of rainfall that peaks close to the noon maximum of insolation, hours earlier than the observed peak. One possible reason is insufficient sensitivity of their cumulus parameterizations to the state of the environment due to weak entrainment. The Weather Research and Forecasting (WRF) model, run at cloud-resolving (600 and 125 m) resolution, is used to study the diurnal transition from shallow to deep convection during the monsoon break period of the Tropical Warm Pool-International Cloud Experiment. The WRF model develops a transition from shallow to deep convection in isolated events by 1430-1500 local time. The inferred entrainment rate weakens with increasing time of day as convection deepens. Several current cumulus parameterizations are tested for their ability to reproduce theWRFbehavior. The Gregory parameterization, in which entrainment rate varies directly with parcel buoyancy and inversely as the square of the updraft speed, is the best predictor of the inferred WRF entrainment profiles. The Gregory scheme depends on a free parameter that represents the fraction of buoyant turbulent kinetic energy generation on the cloud scale that is consumed by the turbulent entrainment process at smaller scales. A single vertical profile of this free parameter, increasing with height above the boundary layer but constant with varying convection depth, produces entrainment rate profiles consistent with those inferred from the WRF over the buoyant depth of the convection. Parameterizations in which entrainment varies inversely with altitude or updraft speed or increases with decreasing tropospheric relative humidity do not perform as well. Entrainment rate at cloud base decreases as convection depth increases; this behavior appears to be related to an increase in vertical velocity at downdraft cold pool edges. Source

Dudek J.C.,NASA
AIAA Journal | Year: 2011

A source-term model that simulates the effects of vortex generators was implemented into the Wind-US Navier- Stokes code. The source term added to the Navier-Stokes equations simulates the lift force that would result from a vane-type vortex generator in the flowfield. The implementation is user-friendly, requiring the user to specify only three quantities for each desired vortex generator: the range of grid points over which the force is to be applied and the planform area and angle of incidence of the physical vane. The model behavior was evaluated for subsonic flow in a rectangular duct with a single vane vortex generator, subsonic flow in an S-duct with 22 corotating vortex generators, and supersonic flow in a rectangular duct with a counter-rotating vortex-generator pair. The model was also used to successfully simulate microramps in supersonic flow by treating each microramp as a pair of vanes with opposite angles of incidence. The validation results indicate that the source-term vortex-generator model provides a useful tool for screening vortex-generator configurations and gives comparable results to solutions computed using gridded vanes. Copyright Clearance Center, Inc. Source

Moore J.M.,NASA | Howard A.D.,University of Virginia
Geophysical Research Letters | Year: 2010

Features observed in the low-latitude basins of Hotei Regio and Tui Regio on Titan have attracted the attention of the Cassini-era investigators. At both locations, Visual Infrared Mapping Spectrometer (VIMS) observed isolated 5-μm bright ∼500 km wide features described as lobate in shape. Several studies have proposed that these materials are cryo-volcanic flows. We propose an alternative explanation. Recently published topographic profiles across Hotei Regio and Tui Regio indicate these features appear to occur in large regional basins, at least along the direction of the profiles. Cassini Synthetic Aperture Radar (SAR) images show that the terrains surrounding both topographically low-lying 5-μm bright features exhibit fluvial networks that appear to converge into the probable basins. The 5-μm bright features themselves correspond to fields of discrete radar-bright depressions whose bounding edges are commonly rounded and cumulate in planform in SAR images. These fields of discrete radar-bright depressions strongly resemble fields of features seen at Titan's high latitudes identified as dry lakes. Thus the combination of (1) the resemblance to high-latitude dry lakes, (2) location in the centers of regional depressions, and (3) convergence of fluvial networks are inferred by us to best explain the features of Hotei Regio and Tui Regio as sites of paleolake clusters (and perhaps former, now dry seas). These low-latitude paleolake clusters or former seas, if real, may be evidence of substantially larger inventories of liquid alkanes in Titan's past. Copyright 2010 by the American Geophysical Union. Source

Daily swath MODIS Terra Collection 6 fractional snow cover (MOD10_L2) estimates were validated with two-day Landsat TM/ETM+snow-covered area estimates across central Idaho and southwestern Montana, USA. Snow cover maps during spring snowmelt for 2000, 2001, 2002, 2003, 2005, 2007, and 2009 were compared between MODIS Terra and Landsat TM/ETM+using least-squared regression. Strong spatial and temporal map agreement was found between MODIS Terra fractional snow cover and Landsat TM/ETM+snow-covered area, although map disagreement was observed for two validation dates. High-altitude cirrus cloud contamination during low snow conditions as well as late season transient snowfall resulted in map disagreement. MODIS Terra's spatial resolution limits retrieval of thin-patchy snow cover, especially during partially cloudy conditions. Landsat's image acquisition frequency can introduce difficulty when discriminating between transient and resident mountain snow cover. Furthermore, transient snowfall later in the snowmelt season, which is a stochastic accumulation event that does not usually persist beyond the daily timescale, will skew decadal snow-covered area variability if bi-monthly climate data record development is the objective. As a quality control step, ground-based daily snow telemetry snow-water-equivalent measurements can be used to verify transient snowfall events. Users of daily MODIS Terra fractional snow products should be aware that local solar illumination and sensor viewing geometry might influence fractional snow cover estimation in mountainous terrain. Cross-sensor interoperability has been confirmed between MODIS Terra and Landsat TM/ETM+when mapping snow from the visible/infrared spectrum. This relationship is strong and supports operational multi-sensor snow cover mapping, specifically climate data record development to expand cryosphere, climate, and hydrological science applications. © 2013 John Wiley & Sons, Ltd. Source

Vander Wal R.L.,Pennsylvania State University | Bryg V.M.,NASA | Hays M.D.,U.S. Environmental Protection Agency
Analytical Chemistry | Year: 2011

Carbonaceous aerosols can vary in elemental content, surface chemistry, and carbon nanostructure. Each of these properties is related to the details of soot formation. Fuel source, combustion process (affecting formation and growth conditions), and postcombustion exhaust where oxidation occurs all contribute to the physical structure and surface chemistry of soot. Traditionally such physical and chemical parameters have been measured separately by various techniques. Presented here is the unified measurement of these characteristics using X-ray photoelectron spectroscopy (XPS). In the present study, XPS is applied to combustion soot collected from a diesel engine (running biodiesel and pump-grade fuels); jet engine; and institutional, plant, and residential oil-fired boilers. Elemental composition is mapped by a survey scan over a broad energy range. Surface chemistry and carbon nanostructure are quantified by deconvolution of high-resolution scans over the C1s region. This combination of parameters forms a distinct matrix of identifiers for the soots from these sources. © 2011 American Chemical Society. Source

Hui H.,University of Notre Dame | Peslier A.H.,Jacobs Engineering | Peslier A.H.,NASA | Zhang Y.,University of Michigan | Neal C.R.,University of Notre Dame
Nature Geoscience | Year: 2013

The Moon was thought to be anhydrous since the Apollo era1, but this view has been challenged by detections of water on the lunar surface 2-4 and in volcanic rocks5-9 and regolith10. Part of this water is thought to have been brought through solar-wind implantation2-4,7,10 and meteorite impacts2,3,7,11, long after the primary lunar crust formed from the cooling magma ocean 12,13. Here we show that this primary crust of the Moon contains significant amounts of water. We analysed plagioclase grains in lunar anorthosites thought to sample the primary crust, obtained in the Apollo missions, using Fourier-transform infrared spectroscopy, and detected approximately 6 ppm water. We also detected up to 2.7 ppm water in plagioclase grains in troctolites also from the lunar highland upper crust. From these measurements, we estimate that the initial water content of the lunar magma ocean was approximately 320 ppm; water accumulating in the final residuum of the lunar magma ocean could have reached 1.4 wt%, an amount sufficient to explain water contents measured in lunar volcanic rocks. The presence of water in the primary crust implies a more prolonged crystallization of the lunar magma ocean than a dry moon scenario and suggests that water may have played a key role in the genesis of lunar basalts. Copyright © 2013 Macmillan Publishers Limited. Source

Archibald R.F.,McGill University | Kaspi V.M.,McGill University | Beardmore A.P.,University of Leicester | Gehrels N.,NASA | Kennea J.A.,Pennsylvania State University
Astrophysical Journal | Year: 2015

PSR J1846-0258 is an object that straddles the boundary between magnetars and rotation powered pulsars. Though behaving for many years as a rotation-powered pulsar, in 2006, it exhibited distinctly magnetar-like behavior - emitting several short hard X-ray bursts, and a flux increase. Here we report on 7 years of post-outburst timing observations of PSR J1846-0258 using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure the braking index over the post-magnetar outburst period to be n = 2.19 ± 0.03. This represents a change of Δn = -0.46 ± 0.03 or a 14.5σ difference from the pre-outburst braking index of n = 2.65 ± 0.01, which itself was measured over a span of 6.5 years. A change to a pulsar braking index so large and long-lived is unprecedented and poses a significant challenge to models of pulsar spin-down. © 2015. The American Astronomical Society. All rights reserved. Source

Cullather R.I.,University of Maryland College Park | Bosilovich M.G.,NASA
Journal of Climate | Year: 2012

Components of the atmospheric energy budget from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) are evaluated in polar regions for the period 1979-2005 and compared with previous estimates, in situ observations, and contemporary reanalyses. Closure of the budget is reflected by the analysis increments term, which indicates an energy surplus of 11 W m -2 over the North Polar cap (70°-90°N) and 22 W m -2 over the South Polar cap (70°-90°S). Total atmospheric energy convergence from MERRA compares favorably with previous studies for northern high latitudes but exceeds the available previous estimate for the South Polar cap by 46%. Discrepancies with the Southern Hemisphere energy transport are largest in autumn and may be related to differences in topography with earlier reanalyses. For the Arctic, differences betweenMERRAand other sources in top of atmosphere (TOA) and surface radiative fluxes are largest in May. These differences are concurrent with the largest discrepancies between MERRA parameterized and observed surface albedo. For May, in situ observations of the upwelling shortwave flux in the Arctic are 80 W m -2 larger than MERRA, while the MERRA downwelling longwave flux is underestimated by 12 W m -2 throughout the year. Over grounded ice sheets, the annual mean net surface energy flux inMERRAis erroneously nonzero. Contemporary reanalyses from the Climate Forecast Center (CFSR) and the Interim Re-Analyses of the European Centre for Medium-Range Weather Forecasts (ERA-I) are found to have better surface parameterizations; however, these reanalyses also disagree with observed surface and TOA energy fluxes. Discrepancies among available reanalyses underscore the challenge of reproducing credible estimates of the atmospheric energy budget in polar regions. © 2012 American Meteorological Society. Source

Vrieling A.,University of Twente | de Beurs K.M.,University of Oklahoma | Brown M.E.,NASA
Climatic Change | Year: 2011

Food security exists when people have access to sufficient, safe and nutritious food at all times to meet their dietary needs. The natural resource base is one of the many factors affecting food security. Its variability and decline creates problems for local food production. In this study we characterize for sub-Saharan Africa vegetation phenology and assess variability and trends of phenological indicators based on NDVI time series from 1982 to 2006. We focus on cumulated NDVI over the season (cumNDVI) which is a proxy for net primary productivity. Results are aggregated at the level of major farming systems, while determining also spatial variability within farming systems. High temporal variability of cumNDVI occurs in semiarid and subhumid regions. The results show a large area of positive cumNDVI trends between Senegal and South Sudan. These correspond to positive CRU rainfall trends found and relate to recovery after the 1980's droughts. We find significant negative cumNDVI trends near the south-coast of West Africa (Guinea coast) and in Tanzania. For each farming system, causes of change and variability are discussed based on available literature (Appendix A). Although food security comprises more than the local natural resource base, our results can perform an input for food security analysis by identifying zones of high variability or downward trends. Farming systems are found to be a useful level of analysis. Diversity and trends found within farming system boundaries underline that farming systems are dynamic. © 2011 The Author(s). Source

Cylinder wakes have been studied extensively over several decades to better understand the basic flow phenomena encountered in such flows. The physics of the very near wake of the cylinder is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. A study of the instability of the detached shear layers is important because these shear layers have a considerable impact on the dynamics of the very near wake. It has been observed experimentally that during certain periods of time that are randomly distributed, the measured fluctuating velocity component near the shear layers shows considerable amplification and it subsequently returns to its normal level (intermittency). Here, direct numerical simulations are used to accomplish a number of objectives such as confirming the presence of intermittency (computationally) and shedding light on processes that contribute significantly to intermittency and shear-layer transition/breakdown. Velocity time traces together with corresponding instantaneous vorticity contours are used in deciphering the fundamental processes underlying intermittency and shear-layer transition. The computed velocity spectra at three locations along the shear layer are provided. The computed shear-layer frequency agrees well with a power-law fit to experimental data. © 2010 Cambridge University Press. Source

Timokhin A.N.,NASA | Timokhin A.N.,University of California at Berkeley | Timokhin A.N.,Moscow State University | Arons J.,University of California at Berkeley
Monthly Notices of the Royal Astronomical Society | Year: 2013

We report the results of an investigation of particle acceleration and electron-positron plasma generation at low altitude in the polar magnetic flux tubes of rotation-powered pulsars, when the stellar surface is free to emit whatever charges and currents are demanded by the forcefree magnetosphere. We apply a new 1D hybrid plasma simulation code to the dynamical problem, using Particle-in-Cell methods for the dynamics of the charged particles, including a determination of the collective electrostatic fluctuations in the plasma, combined with a Monte Carlo treatment of the high-energy gamma-rays that mediate the formation of the electron- positron pairs.We assume the electric current flowing through the pair creation zone is fixed by the much higher inductance magnetosphere, and adopt the results of force-free magnetosphere models to provide the currents which must be carried by the accelerator. The models are spatially one dimensional, and designed to explore the physics, although of practical relevance to young, high-voltage pulsars. We observe novel behaviour (a)When the current density j is less than the Goldreich-Julian value (0 < j/jGJ < 1), space charge limited acceleration of the current carrying beam is mild, with the full Goldreich-Julian charge density comprising the charge densities of the beam and a cloud of electrically trapped particles with the same sign of charge as the beam. The voltage drops are of the order of mc2/e, and pair creation is absent. (b) When the current density exceeds the Goldreich-Julian value (j/jGJ > 1), the system develops high voltage drops (TV or greater), causing emission of curvature gamma-rays and intense bursts of pair creation. The bursts exhibit limit cycle behaviour, with characteristic time-scales somewhat longer than the relativistic fly-by time over distances comparable to the polar cap diameter (microseconds). (c) In return current regions, where j/jGJ < 0, the system develops similar bursts of pair creation. These discharges are similar to those encountered in previous calculations by Timokhin of pair creation when the surface has a high work function and cannot freely emit charge. In cases (b) and (c), the intermittently generated pairs allow the system to simultaneously carry the magnetospherically prescribed currents and adjust the charge density and average electric field to force-free conditions. We also elucidate the conditions for pair creating beam flow to be steady (stationary with small fluctuations in the rotating frame), finding that such steady flows can occupy only a small fraction of the current density parameter space exhibited by the forcefree magnetospheric model. The generic polar flow dynamics and pair creation are strongly time dependent. The model has an essential difference from almost all previous quantitative studies, in that we sought the accelerating voltage (with pair creation, when the voltage drops are sufficiently large; without, when they are small)as a function of the applied current.The 1D results described here characterize the dependence of acceleration and pair creation on the magnitude and sign of current. The dependence on the spatial distribution of the current is amulti-dimensional problem, possibly exhibitingmore chaotic behaviour.We briefly outline possible relations of the electric field fluctuations observed in the polar flows (both with and without pair creation discharges) to direct emission of radio waves, as well as revive the possible relation of the observed limit cycle behaviour to microstructure in the radio emission. Actually modelling these effects requires the multi-dimensional treatment, to be reported in a later paper ©2012 The Authors. Source

Schuet S.,NASA
IEEE Sensors Journal | Year: 2010

A new method for detecting and locating wiring damage using time domain reflectometry with arbitrary input interrogation signals is presented. This method employs existing ℓ1 regularization techniques from convex optimization and compressed sensing to exploit sparsity in the distribution of faults along the length of a wire, while further generalizing and improving commonly used fault detection techniques based on sliding correlation and peak detection. The method's effectiveness is demonstrated using a simulated example, and it is shown how Monte Carlo techniques are used to tune it to achieve specific detection goals, like a certain false positive error rate. Furthermore, the method is easily implemented by adapting readily available optimization algorithms to quickly solve large, high resolution, versions of this estimation problem. Finally, the technique is applied to a real data set, which reveals its impressive ability to identify a subtle type of chafing damage on real wire. © 2010 IEEE. Source

Sears D.W.G.,NASA
Space Science Reviews | Year: 2015

Interest in asteroids is currently high in view of their scientific importance, the impact hazard, and the in situ resource opportunities they offer. They are also a case study of the intimate relationship between science and exploration. A detailed review of the twelve asteroids that have been visited by eight robotic spacecraft is presented here. While the twelve explored asteroids have many features in common, like their heavily cratered and regolith covered surfaces, they are a remarkably diverse group. Some have low-eccentricity orbits in the main belt, while some are potentially hazardous objects. They range from dwarf planets to primary planetesimals to fragments of larger precursor objects to tiny shards. One has a moon. Their surface compositions range from basaltic to various chondrite-like compositions. Here their properties are reviewed and what was confirmed and what was newly learned is discussed, and additionally the explored asteroids are compared with comets and meteorites. Several topics are developed. These topics are the internal structure of asteroids, water distribution in the inner solar system and its role in shaping surfaces, and the meteoritic links. It is suggested, that asteroid-scale grooves, ridges, and catenas on several explored asteroids argue against these asteroids having rubble pile interiors, i.e. interiors made when impact fragments reaccumulate. The only body for which this is not true is the tiny Itokawa and it is argued that this asteroid is a regolith breccia. The discovery of water on Vesta, fluidization textures on comets and possibly Eros, and the relatively large number of active asteroids inside the purported snowline, suggests that significant subsurface water may be present on asteroids in the inner solar system and may partly account for their low densities. The explored asteroids have also confirmed the linkage of the HED meteorites with Vesta and Itokawa with the ordinary chondrite meteorites, Eros is somewhat problematical. So while diversity, and the range of sizes, histories, and surface compositions, is the hall mark of the explored asteroids, the number of explored asteroids is small compared with the diversity of material expected on the basis of asteroid astronomy and meteorite geochemistry. The exploration of the solar system’s asteroids has only just begun. © 2015, Springer Science+Business Media Dordrecht. Source

Sibonga J.D.,NASA
Current Osteoporosis Reports | Year: 2013

Currently, the measurement of areal bone mineral density (aBMD) is used at NASA to evaluate the effects of spaceflight on the skeletal health of astronauts. Notably, there are precipitous declines in aBMD with losses >10 % detected in the hip and spine in some astronauts following a typical 6-month mission in space. How those percentage changes in aBMD relate to fracture risk in the younger-aged astronaut is unknown. Given the unique set of risk factors that could be contributing to this bone loss (eg, adaptation to weightlessness, suboptimal diet, reduced physical activity, perturbed mineral metabolism), one might not expect skeletal changes due to spaceflight to be similar to skeletal changes due to aging. Consequently, dual-energy X-ray absorptiometry (DXA) measurement of aBMD may be too limiting to understand fracture probability in the astronaut during a long-duration mission and the risk for premature osteoporosis after return to Earth. Following a brief review of the current knowledge-base, this paper will discuss some innovative research projects being pursued at NASA to help understand skeletal health in astronauts. © 2013 Springer Science+Business Media New York (outside the USA). Source

Rogers B.M.,University of California at Irvine | Rogers B.M.,Woods Hole Oceanographic Institution | Soja A.J.,NASA | Goulden M.L.,University of California at Irvine | Randerson J.T.,University of California at Irvine
Nature Geoscience | Year: 2015

Wildfires are common in boreal forests around the globe and strongly influence ecosystem processes. However, North American forests support more high-intensity crown fires than Eurasia, where lower-intensity surface fires are common. These two types of fire can result in different net effects on climate as a consequence of their contrasting impacts on terrestrial albedo and carbon stocks. Here we use remote-sensing imagery, climate reanalysis data and forest inventories to evaluate differences in boreal fire dynamics between North America and Eurasia and their key drivers. Eurasian fires were less intense, destroyed less live vegetation, killed fewer trees and generated a smaller negative shortwave forcing. As fire weather conditions were similar across continents, we suggest that different fire dynamics between the two continents resulted from their dominant tree species. In particular, species that have evolved to spread and be consumed by crown fires as part of their life cycle dominate North American boreal forests. In contrast, tree species that have evolved to resist and suppress crown fires dominate Eurasian boreal forests. We conclude that species-level traits must be considered in global evaluations of the effects of fire on emissions and climate. © 2015 Macmillan Publishers Limited. Source

Leib S.J.,Ohio Aerospace Institute | Goldstein M.E.,NASA
AIAA Journal | Year: 2011

This paper introduces a novel hybrid source model into an existing acoustic analogy approach to obtain improved predictions of the turbulent mixing noise from cold, round, subsonic, and supersonic jets. The model incorporates new features of the Reynolds stress autocovariance tensor components found in recent experiments. The model parameters are determined from a Reynolds-averaged Navier-Stokes flow solution and experimental data. It is shown that this model significantly improves the predictions relative to previous results, particularly at observer polar angles between 90 degrees to the jet axis and the peak noise direction, indicating the importance of properly modeling relatively subtle characteristics of the autocovariance functions. The results are used to infer the relative importance of individual terms that make up the formula for the acoustic spectrum as a function of jet Mach number, frequency, and observer location. Source

Borucki W.J.,NASA
Reports on Progress in Physics | Year: 2016

The Kepler Mission is a space observatory launched in 2009 by NASA to monitor 170 000 stars over a period of four years to determine the frequency of Earth-size and larger planets in and near the habitable zone of Sun-like stars, the size and orbital distributions of these planets, and the types of stars they orbit. Kepler is the tenth in the series of NASA Discovery Program missions that are competitively-selected, PI-directed, medium-cost missions. The Mission concept and various instrument prototypes were developed at the Ames Research Center over a period of 18 years starting in 1983. The development of techniques to do the 10 ppm photometry required for Mission success took years of experimentation, several workshops, and the exploration of many 'blind alleys' before the construction of the flight instrument. Beginning in 1992 at the start of the NASA Discovery Program, the Kepler Mission concept was proposed five times before its acceptance for mission development in 2001. During that period, the concept evolved from a photometer in an L2 orbit that monitored 6000 stars in a 50 sq deg field-of-view (FOV) to one that was in a heliocentric orbit that simultaneously monitored 170 000 stars with a 105 sq deg FOV. Analysis of the data to date has detected over 4600 planetary candidates which include several hundred Earth-size planetary candidates, over a thousand confirmed planets, and Earth-size planets in the habitable zone (HZ). These discoveries provide the information required for estimates of the frequency of planets in our galaxy. The Mission results show that most stars have planets, many of these planets are similar in size to the Earth, and that systems with several planets are common. Although planets in the HZ are common, many are substantially larger than Earth. © 2016 IOP Publishing Ltd. Source

Chan W.M.,NASA
20th AIAA Computational Fluid Dynamics Conference 2011 | Year: 2011

Recent developments in strategies and software tools for overset structured grid generation and domain connectivity are described. A scripting approach based on a library of grid generation script macros is adopted to speed up the grid generation process. Advances in this method involve the development of high level functions that encapsulate more complex grid generation tasks. Sample applications using the script library macros are given for several complex configurations. Advances towards automated and effcient domain connectivity is accomplished using a newly developed library of domain connectivity functions called Chimera Components Connectivity Library (C3LIB). The library contains standard interfaces to the various processes encountered in domain connectivity. Enhancement to the stencil search speed is accomplished via bounding boxes based on balanced index-space partitions, and replacement of global searches by local searches using space-filling curve segments. The performance of the parallel stencil search algorithm is illustrated via a set of test cases on practical complex configurations. Source

Zaretsky E.V.,NASA
Materials Science and Technology | Year: 2012

Starting about 1920 it becomes easier to track the growth of bearing materials technology. Until 1955, with few exceptions, comparatively little progress was made in this area. AISI 52100 and some carburising grades (AISI 4320, AISI 9310) were adequate for most applications. The catalyst to quantum advances in high-performance rolling-element bearing steels was the advent of the aircraft gas turbine engine. With improved bearing manufacturing and steel processing together with lubrication technology, the potential improvements in bearing life can as much as 80 times that attainable in the late 1950s or as much as 400 times that attainable in 1940. This paper summarises the chemical, metallurgical and physical aspects of bearing steels and their effect on rolling bearing life and reliability: the single most important variable that has significantly increased bearing life and reliability is vacuum processing of bearing steel. Differences between through hardened, case carburised and corrosion resistant steels are discussed. The interrelation of alloy elements and carbides and their effect on bearing life are presented. An equation relating bearing life, steel hardness and temperature is given. Life factors for various steels are suggested and discussed. A relation between compressive residual stress and bearing life is presented. The effects of retained austenite and grain size are discussed. © 2012 Institute of Materials, Minerals and Mining. Source

Angerhausen D.,NASA
Astrophysical Journal | Year: 2015

Trojans are small bodies in planetary Lagrangian points. In our solar system, Jupiter has the largest number of such companions. Their existence is assumed for exoplanetary systems as well, but none have been found so far. We present an analysis by super-stacking ∼4 × 103 Kepler planets with a total of ∼9 × 104 transits, searching for an average Trojan transit dip. Our results give an upper limit to the average Trojan transiting area (per planet) that corresponds to one body of radius with confidence. We find a significant Trojan-like signal in a sub-sample for planets with more (or larger) Trojans for periods >60 days. Our tentative results can and should be checked with improved data from future missions like PLATO 2.0, and can guide planetary formation theories. © 2015. The American Astronomical Society. All rights reserved. Source

Lau K.-M.,NASA | WU H.-T.,Science Systems And Applications Inc.
Journal of Climate | Year: 2010

This study investigates the evolution of cloud and rainfall structures associated with Madden-Julian oscillation (MJO) using Tropical Rainfall Measuring Mission (TRMM) data. Two complementary indices are used to define MJO phases. Joint probability distribution functions (PDFs) of cloud-top temperature and radar echo-top height are constructed for each of the eight MJO phases. The genesis stage of MJO convection over the western Pacific (phases 1 and 2) features a bottom-heavy PDF, characterized by abundant warm rain, low clouds, suppressed deep convection, and higher sea surface temperature (SST). As MJO convection develops (phases 3 and 4), a transition from the bottom-heavy to top-heavy PDF occurs. The latter is associated with the development of mixed-phase rain and middle-to-high clouds, coupled with rapid SST cooling. At the MJO convection peak (phase 5), a top-heavy PDF contributed by deep convection with mixed-phase and ice-phase rain and high echo-top heights (>5 km) dominates. The decaying stage (phases 6 and 7) is characterized by suppressed SST, reduced total rain, increased contribution from stratiform rain, and increased nonraining high clouds. Phase 7, in particular, signals the beginning of a return to higher SST and increased warm rain. Phase 8 completes the MJO cycle, returning to a bottom-heavy PDF and SST conditions similar to phase 1. The structural changes in rain and clouds at different phases of MJO are consistent with corresponding changes in derived latent heating profiles, suggesting the importance of a diverse mix of warm, mixed-phase, and ice-phase rain associated with low-level, congestus, and high clouds in constituting the life cycle and the time scales of MJO. Source

Lau W.K.M.,NASA | Kim K.-M.,University of Maryland Baltimore County
Geophysical Research Letters | Year: 2010

In this paper, we present corroborative observational evidences from satellites, in-situ observations, and re-analysis data showing possible impacts of absorbing aerosols on subseasonal and regional summer monsoon rainfall over India. We find that increased absorbing aerosols in the Indo-Gangetic Plain in recent decades may have led to long-term warming of the upper troposphere over northern India and the Tibetan Plateau, enhanced rainfall in northern India and the Himalayas foothill regions in the early part (May-June) of the monsoon season, followed by diminished rainfall over central and southern Indian in the latter part (July-August) of the monsoon season. These signals, which are consistent with current theories of atmospheric heating and solar dimming by aerosol and induced cloudiness in modulating the Indian monsoon, would have been masked by conventional method of using all-India rainfall averaged over the entire monsoon season. © 2010 by the American Geophysical Union. Source

Koshak W.J.,NASA
Journal of Atmospheric and Oceanic Technology | Year: 2010

Flashes detected by the Optical Transient Detector (OTD) that occur over the continental United States (CONUS) are intercompared with data from the National Lightning Detection Network (NLDN) in order to partition the OTD flashes into ground and cloud flashes. The entire 5-yr OTD dataset for CONUS is analyzed. The statistical distributions of a variety of optical characteristics are examined, including five flash-level attributes (radiance, area, duration, number of optical groups, and number of optical events), and two group-level attributes [the maximum number of events in a group (MNEG), and a closely related parameter, the maximum group area (MGA)]. On average, there were 5.6 optical groups per return stroke in a ground flash, which is in part due to the likelihood that OTD detects interstroke K changes. It was found that return strokes within ground flashes typically produce large optical groups; hence, the MNEG and MGA parameters serve as useful "return-stroke detectors." The results of this study provide insight on how to construct an algorithm for retrieving the fraction of ground flashes in a set of flashes observed from a satellite lightning imager. Specifically, even though it is shown that the statistical distributions of the optical characteristics for ground and cloud flashes overlap substantially, the mean values of these distributions differ. Hence, a retrieval method that is based on an analysis of the distribution of the means, and that employs the central limit theorem of statistics, is recommended. As the sample size used to compute the means is increased, the overlap in the distributions of the means for ground and cloud flashes is diminished, making ground flash fraction retrieval feasible. Of the seven optical characteristics examined here, the mean MNEG and mean MGA parameters are suggested as being the most useful for discriminating between ground and cloud flashes in the context of this "central limit theorem" approach. © 2010 American Meteorological Society. Source

Shebalin J.V.,NASA
Physics of Plasmas | Year: 2010

Two-dimensional (2D) homogeneous magnetohydrodynamic (MHD) turbulence has many of the same qualitative features as three-dimensional (3D) homogeneous MHD turbulence. These features include several ideal (i.e., nondissipative) invariants along with the phenomenon of broken ergodicity (defined as nonergodic behavior over a very long time). Broken ergodicity appears when certain modes act like random variables with mean values that are large compared to their standard deviations, indicating a coherent structure or dynamo. Recently, the origin of broken ergodicity in 3D MHD turbulence that is manifest in the lowest wavenumbers was found. Here, we study the origin of broken ergodicity in 2D MHD turbulence. It will be seen that broken ergodicity in ideal 2D MHD turbulence can be manifest in the lowest wavenumbers of a finite numerical model for certain initial conditions or in the highest wavenumbers for another set of initial conditions. The origins of broken ergodicity in an ideal 2D homogeneous MHD turbulence are found through an eigenanalysis of the covariance matrices of the probability density function and by an examination of the associated entropy functional. When the values of ideal invariants are kept fixed and grid size increases, it will be shown that the energy in a few large modes remains constant, while the energy in any other mode is inversely proportional to grid size. Also, as grid size increases, we find that broken ergodicity becomes manifest at more and more wavenumbers. © 2010 U.S. Government. Source

Sippel J.A.,NASA | Zhang F.,Pennsylvania State University
Journal of the Atmospheric Sciences | Year: 2010

This study uses ensemble Kalman filter analyses and short-range ensemble forecasts to study factors affecting the predictability of Hurricane Humberto, which made landfall along the Texas coast in 2007. Humberto is known for both its rapid intensification and extreme forecast uncertainty, which makes it an ideal case in which to examine the origins of tropical cyclone strength forecast error. Statistical correlation is used to determine why some ensemble members strengthen the incipient low into a hurricane and others do not. During the analysis period, it is found that variations in midlevel moisture, low-level convective instability, and strength of a front to the north of the cyclone likely lead to differences in net precipitation, which ultimately leads to storm strength spread. Stronger storms are favored when the atmosphere is more moist and unstable and when the front is weaker, possibly because some storms in the ensemble begin entraining cooler and drier postfrontal air during this period. Later during the free forecast, variable entrainment of postfrontal air becomes a leading cause of strength spread. Surface moisture differences are the primary contributor to intensity forecast differences, and convective instability differences play a secondary role. Eventually mature tropical cyclone dynamics and differences in landfall time result in very rapid growth of ensemble spread. These results are very similar to a previous study that investigated a 2004 Gulf of Mexico low with a different model and analysis technique, which gives confidence that they are relevant to tropical cyclone formation and intensification in general. Finally, the rapid increase in forecast uncertainty despite relatively modest differences in initial conditions highlights the need for ensembles and advanced data assimilation techniques. © 2010 American Meteorological Society. Source

Bala G.,Indian Institute of Science | Caldeira K.,Carnegie Institution | Nemani R.,NASA
Climate Dynamics | Year: 2010

Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO2 forcing of similar magnitude. In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen's "fixed-SST forcing" method. Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed. © 2009 Springer-Verlag. Source

The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL's properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ~700 hPa), but moisten the air at midlevels. Therefore, mid- to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet. Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained. © 2010 American Meteorological Society. Source

Fenton L.K.,NASA | Hayward R.K.,U.S. Geological Survey
Geomorphology | Year: 2010

In a study area spanning the martian surface poleward of 50° S., 1190 dune fields have been identified, mapped, and categorized based on dune field morphology. Dune fields in the study area span ∼ 116400km2, leading to a global dune field coverage estimate of ∼904000km2, far less than that found on Earth. Based on distinct morphological features, the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding. The six dune field classes occur in specific latitude zones, with a sequence of reduced activity and degradation progressing poleward. In particular, the first signs of stabilization appear at ∼60° S., which broadly corresponds to the edge of high concentrations of water-equivalent hydrogen content (observed by the Neutron Spectrometer) that have been interpreted as ground ice. This near-surface ground ice likely acts to reduce sand availability in the present climate state on Mars, stabilizing high latitude dunes and allowing erosional processes to change their morphology. As a result, climatic changes in the content of near-surface ground ice are likely to influence the level of dune activity. Spatial variation of dune field classes with longitude is significant, suggesting that local conditions play a major role in determining dune field activity level. Dune fields on the south polar layered terrain, for example, appear either potentially active or inactive, indicating that at least two generations of dune building have occurred on this surface. Many dune fields show signs of degradation mixed with crisp-brinked dunes, also suggesting that more than one generation of dune building has occurred since they originally formed. Dune fields superposed on early and late Amazonian surfaces provide potential upper age limits of ∼100My on the south polar layered deposits and ∼3Ga elsewhere at high latitudes. No craters are present on any identifiable dune fields, which can provide a lower age limit through crater counting: assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age, their estimated crater retention age is <∼10000years. An average-sized uncratered dune field (94km2) has a crater retention age <∼8My. This apparent youth suggests that present-day climate conditions are responsible for the observed degradation and reduced level of aeolian activity. A lack of observed transport pathways and the absence of large dune fields in the largest basins (Hellas and Argyre Planitiae) are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on Mars. © 2009 Elsevier B.V. Source

Loeb N.G.,NASA | Su W.,SSAI
Journal of Climate | Year: 2010

To provide a lower bound for the uncertainty inmeasurement-based clear-and all-sky direct aerosol radiative forcing (DARF), a radiative perturbation analysis is performed for the ideal case in which the perturbations in global mean aerosol properties are given by published values of systematic uncertainty in Aerosol Robotic Network (AERONET) aerosol measurements. DARF calculations for base-state climatological cloud and aerosol properties over ocean and land are performed, and then repeated after perturbing individual aerosol optical properties (aerosol optical depth, single-scattering albedo, asymmetry parameter, scale height, and anthropogenic fraction) from their base values, keeping all other parameters fixed. The total DARF uncertainty from all aerosol parameters combined is 0.5-1.0 W m -2, a factor of 2-4 greater than the value cited in the Intergovernmental Panel on Climate Change's (IPCC's) Fourth Assessment Report. Most of the total DARF uncertainty in this analysis is associatedwith single-scattering albedo uncertainty. Owing to the greater sensitivity to single-scattering albedo in cloudy columns, DARF uncertainty in all-sky conditions is greater than in clear-sky conditions, even though the global mean clear-sky DARF is more than twice as large as the all-sky DARF. © 2010 American Meteorological Society. Source

Koren I.,Weizmann Institute of Science | Feingold G.,National Oceanic and Atmospheric Administration | Remer L.A.,NASA
Atmospheric Chemistry and Physics | Year: 2010

Associations between cloud properties and aerosol loading are frequently observed in products derived from satellite measurements. These observed trends between clouds and aerosol optical depth suggest aerosol modification of cloud dynamics, yet there are uncertainties involved in satellite retrievals that have the potential to lead to incorrect conclusions. Two of the most challenging problems are addressed here: the potential for retrieved aerosol optical depth to be cloud-contaminated, and as a result, artificially correlated with cloud parameters; and the potential for correlations between aerosol and cloud parameters to be erroneously considered to be causal. Here these issues are tackled directly by studying the effects of the aerosol on convective clouds in the tropical Atlantic Ocean using satellite remote sensing, a chemical transport model, and a reanalysis of meteorological fields. Results show that there is a robust positive correlation between cloud fraction or cloud top height and the aerosol optical depth, regardless of whether a stringent filtering of aerosol measurements in the vicinity of clouds is applied, or not. These same positive correlations emerge when replacing the observed aerosol field with that derived from a chemical transport model. Model-reanalysis data is used to address the causality question by providing meteorological context for the satellite observations. A correlation exercise between the full suite of meteorological fields derived from model reanalysis and satellite-derived cloud fields shows that observed cloud top height and cloud fraction correlate best with model pressure updraft velocity and relative humidity. Observed aerosol optical depth does correlate with meteorological parameters but usually different parameters from those that correlate with observed cloud fields. The result is a near-orthogonal influence of aerosol and meteorological fields on cloud top height and cloud fraction. The results strengthen the case that the aerosol does play a role in invigorating convective clouds. © 2010 Author(s). Source

Since 1963 more than 900 spacecraft and more than 200 launch vehicle upper stages have been inserted into the vicinity of the geosynchronous regime. Equally important, more than 300 spacecraft have been maneuvered into disposal orbits at mission termination to alleviate unnecessary congestion in the finite GEO region. However, the number of GEO satellites continues to grow, and evidence exists of a substantial small debris population. In addition, the operational modes of an increasing number of GEO spacecraft differ from those of their predecessors of several decades ago, including more frequent utilization of inclined and eccentric geosynchronous orbits. Consequently, the nature of the GEO regime and its immediate surroundings is evolving from well-known classical characteristics. This paper takes a fresh look at the GEO satellite population and the near- and far-term environmental implications of the region, including the effects of national and international debris mitigation measures. Source

Remsberg E.,NASA
Journal of Geophysical Research: Atmospheres | Year: 2010

The 14 year (1991-2005) time series of mesospheric water vapor from the Halogen Occultation Experiment (HALOE) are analyzed using multiple linear regression (MLR) techniques for their seasonal and longer-period terms from 45°S to 45°N. The distribution of annual average water vapor shows a decrease from a maximum of 6.5 ppmv at 0.2 hPa to about 3.2 ppmv at 0.01 hPa, in accord with the effects of the photolysis of water vapor due to the Lyman-α flux. The distribution of the semiannual cycle amplitudes is nearly hemispherically symmetric at the low latitudes, while that of the annual cycles shows larger amplitudes in the Northern Hemisphere. The diagnosed 11 year, or solar cycle, max minus min, water vapor values are of the order of several percent at 0.2 hPa to about 23% at 0.01 hPa. The solar cycle terms have larger values in the Northern than in the Southern Hemisphere, particularly in the middle mesosphere, and the associated linear trend terms are anomalously large in the same region. Those anomalies are due, at least in part, to the fact that the amplitudes of the seasonal cycles were varying at northern midlatitudes during 1991-2005, while the corresponding seasonal terms of the MLR model do not allow for that possibility. Although the 11 year variation in water vapor is essentially hemispherically symmetric and antiphased with the solar cycle flux near 0.01 hPa, the concurrent temperature variations produce slightly colder conditions at the northern high latitudes at solar minimum. It is concluded that this temperature difference is most likely the reason for the greater occurrence of polar mesospheric clouds at the northern versus the southern high latitudes at solar minimum during the HALOE time period. Copyright 2010 by the American Geophysical Union. Source

Herman J.R.,University of Maryland Baltimore County | Herman J.R.,NASA
Journal of Geophysical Research: Atmospheres | Year: 2010

Multiple scattering radiative transfer results are used to calculate action spectrum weighted irradiances and fractional irradiance changes in terms of a power law in Ω ozone , U(Ω/200)-RAF, where the new radiation amplification factor (RAF) is just a function of solar zenith angle. Including Rayleigh scattering caused small differences in the estimated 30 year changes in action spectrum-weighted irradiances compared to estimates that neglect multiple scattering. The radiative transfer results are applied to several action spectra and to an instrument response function corresponding to the Solar Light 501 meter. The effect of changing ozone on two plant damage action spectra are shown for plants with high sensitivity to UVB (280-315 nm) and those with lower sensitivity, showing that the probability for plant damage for the latter has increased since 1979, especially at middle to high latitudes in the Southern Hemisphere. Similarly, there has been an increase in rates of erythemal skin damage and pre-vitamin D3 production corresponding to measured ozone decreases. An example conversion function is derived to obtain erythemal irradiances and the UV index from measurements with the Solar Light 501 instrument response function. An analytic expressions is given to convert changes in erythemal irradiances to changes in CIE vitamin-D action spectrum weighted irradiances. Copyright 2010 by the American Geophysical Union. Source

Yang Q.,University of Washington | Fu Q.,University of Washington | Hu Y.,NASA
Journal of Geophysical Research: Atmospheres | Year: 2010

We quantify the seasonal and spatial variations of cloud radiative impacts in the tropical tropopause layer (TTL) by using cloud retrievals from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), International Satellite Cloud Climatology Project (ISCCP) and CloudSat. Over the convective regions including Western Pacific, Africa, South America, and South Asia, we find pronounced solar heating and infrared cooling in the lower part of the TTL (<∼16 km). The solar heating weakens above 16 km and nearly diminishes at 18 km, whereas the infrared cooling extends vertically throughout the TTL. The net cloud radiative forcing, which is the summation of cloud solar and infrared radiative forcing, has heating below ∼16 km and turns to mostly cooling above 17 km. The net cloud radiative heating over the convective regions is mainly contributed from solar radiation, whereas the weak net cloud radiative heating surrounding these regions is due to infrared heating. To further examine the impacts of different cloud types in the TTL, we classified TTL clouds in terms of cloud optical depths (τ) as thin cirrus (τ < 0.3), thick cirrus (0.3 ≤ τ < 3), and opaque clouds (τ ≥ 3). In the solar part, thin and thick cirrus play a relatively small role and the impact of cloud-free air above clouds is negligible. The solar heating is dominantly contributed from the solar absorption near the top of opaque clouds. In the infrared part, the thick cirrus heating is mainly confined over the convective regions in the lower part of TTL while the thin cirrus infrared heating is more prevalent both vertically and horizontally in the TTL, which is the dominant infrared heating source. The infrared cooling in cloud-free air above clouds is dominant above 17 km, whereas the infrared cooling near the top of opaque clouds is dominant below. Despite the infrared heating effects of thin and thick cirrus clouds, the infrared cooling from the opaque cloud top and cloud-free air above clouds outweighs the heating effects so that the ensemble mean cloud infrared radiative forcing is mostly cooling except outside the convective regions. Copyright 2010 by the American Geophysical Union. Source

Birkett C.M.,The Interdisciplinary Center | Beckley B.,NASA
Marine Geodesy | Year: 2010

Many inland water investigations utilize archival and near-real time radar altimetry data to enable observation of the variation in surface water level. A multialtimeter approach allows a more global outlook with improved spatial resolution, and combined long-term observations improve statistical analyses. Central to all programs is a performance assessment of each instrument. Here, we focus on data quantity and quality pertaining to the Poseidon-3 radar altimeter onboard the Jason-2/OSTM satellite. Utilizing an interim data set (IGDR), studies show that the new on-board DIODE/median and DIODE/DEM tracking modes are performing well, acquiring and maintaining the majority of lake and reservoir surfaces in varying terrains. The 20-Hz along-track resolution of the data, and particularly the availability of the range output from the ice-retracker algorithm, also improves the number of valid height measurements. Based on test-case lakes and reservoirs, output from the ice-retracker algorithm is also seen to have a clear advantage over the ocean-retracker having better height stability across calm and icy surfaces, a greater ability to gain coastline waters, and less sensitivity to loss of water surface when there is island contamination in the radar echo. Such on-board tracking and postprocessing retracking enables the lake waters to be quickly gained after coastline crossing. Values can range from <0.1 s to 2.5 s, but the majority of measurements are obtained in less than 0.4 s or <2.3 km from the coast. Validation exercises reveal that targets of ∼150 km2 surface area and ∼0.8 km width are able to be monitored offering greater potential to acquire lakes in the 100–300 km2 size-category. Time series of height variations are also found to be accurate to ∼3 to 33 cm rms depending on target size and the presence of winter ice. These findings are an improvement over the IGDR/GDR results from the predecessor Jason-1 and TOPEX/Poseidon missions and can satisfy the accuracy requirements of both the science-related and operational lake study programs. © 2010, Taylor & Francis Group, LLC. Source

Martin R.,NASA
IEEE Transactions on Automatic Control | Year: 2016

This technical note investigates the effect of stability on the probability of correctly predicting level-crossings for the output of a linear dynamical system driven by Gaussian noise. It is found that decreasing the stability margin has a favorable effect on predictive capability. The insight from this finding was derived from a parametric analysis of a given measure of predictive capability, represented as an explicit function of the spectral radius. The formulae used to characterize this relationship were derived under strict technical conditions in previous work. However, as a result of the closed-form nature of these expressions, using these formulae to gain insight on the influence of stability was much more computationally efficient than would otherwise be possible under more relaxed technical conditions. © 1963-2012 IEEE. Source

Hippke M.,Institute for Data Analysis | Angerhausen D.,NASA
Astrophysical Journal | Year: 2015

Time-series transit photometry from the Kepler space telescope has allowed for the discovery of thousands of exoplanets. We explore the potential of yet improved future missions such as PLATO 2.0 in detecting solar system analogs. We use real-world solar data and end-to-end simulations to explore the stellar and instrumental noise properties. By injecting and retrieving planets, rings, and moons of our own solar system, we show that the discovery of Venus and Earth analogs transiting G dwarfs like our Sun is feasible at high signal-to-noise ratio after collecting 6 yr of data, but Mars and Mercury analogs will be difficult to detect owing to stellar noise. In the best cases, Saturn's rings and Jupiter's moons will be detectable even in single-transit observations. Through the high number (>1 billion) of observed stars by PLATO 2.0, it will become possible to detect thousands of single-transit events by cold gas giants, analogs to our Jupiter, Saturn, Uranus, and Neptune. Our own solar system aside, we also show, through signal injection and retrieval, that PLATO 2.0 class photometry will allow for the secure detection of exomoons transiting quiet M dwarfs. This is the first study analyzing in depth the potential of future missions and the ultimate limits of photometry, using realistic case examples. © 2015. The American Astronomical Society. All rights reserved. Source

Since 1982, the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by the National Aeronautics and Space Administration (NASA) as well as national and international programs. The CDDIS provides easy, timely, and reliable access to a variety of data sets, products, and information about these data. These measurements, obtained from a global network of nearly 650 instruments at more than 400 distinct sites, include DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), GNSS (Global Navigation Satellite System), SLR and LLR (Satellite and Lunar Laser Ranging), and VLBI (Very Long Baseline Interferometry). The CDDIS data system and its archive have become increasingly important to many national and international science communities, particularly several of the operational services within the International Association of Geodesy (IAG) and its observing system the Global Geodetic Observing System (GGOS), including the International DORIS Service (IDS), the International GNSS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International Earth rotation and Reference frame Service (IERS). Investigations resulting from the data and products available through the CDDIS support research in many aspects of Earth system science and global change. Each month, the CDDIS archives more than one million data and derived product files totaling over 90 Gbytes in volume. In turn, the global user community downloads nearly 1.2 Tbytes (over 10.5 million files) of data and products from the CDDIS each month. The requirements of analysts have evolved since the start of the CDDIS; the specialized nature of the system accommodates the enhancements required to support diverse data sets and user needs. This paper discusses the CDDIS, including background information about the system and its user communities, archive contents, available metadata, and future plans. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved. Source

Pretorius M.L.,University of Oxford | Mukai K.,NASA | Mukai K.,University of Maryland Baltimore County
Monthly Notices of the Royal Astronomical Society | Year: 2014

We construct a complete, hard X-ray flux-limited sample of intermediate polars (IPs) from the Swift-BAT 70-month survey, by imposing selection cuts in flux and Galactic latitude (FX > 2.5 × 10-11 erg cm-2 s-1 at 14-195 keV, and |b| > 5{ring operator}). We then use it to estimate the space density (ρ) of IPs. Assuming that this sample of 15 long-period systems is representative of the intrinsic IP population, the space density of long-period IPs is 1+1 -0.5 × 10-7 pc-3. The Swift-BAT data also allow us to place upper limits on the size of a hypothetical population of faint IPs that is not included in the flux-limited sample. While most IPs detected by Swift-BAT have 14-195 keV luminosities of ~1033 erg s-1, there is evidence of a fainter population at LX ~ 1031 erg s-1. We find that a population of IPs with this luminosity may have a space density as large as 5 × 10-6 pc-3. Furthermore, these low-luminosity IPs, despite appearing rare in observed samples, are probably at least as intrinsically common as the brighter systems that are better represented in the known IP sample.© 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Nguyen N.T.,NASA
Systems and Control Letters | Year: 2012

In the presence of large uncertainties, a control system needs to be able to adapt rapidly to regain performance. Fast adaptation is referred to the implementation of adaptive control with a large adaptive gain so as to reduce the tracking error rapidly. However, a large adaptive gain can lead to high-frequency oscillations which can adversely affect robustness. A new adaptive law, called optimal control modification, is presented that can achieve robust adaptation with a large adaptive gain without incurring high-frequency oscillations. The modification is based on a minimization of the L 2 norm of the tracking error bounded away from some lower bound, formulated as an optimal control problem. The optimality condition is used to derive the modification based on the Pontryagin's Minimum Principle. The optimal control modification is shown to improve robustness of the standard MRAC without significantly compromising the tracking performance. Flight control simulations demonstrate the effectiveness of the new adaptive law. A series of recent, successful flight tests of this adaptive law on a NASA F/A-18A aircraft at NASA Dryden Flight Research Center further demonstrate the effectiveness of the optimal control modification adaptive law. © 2012 Elsevier B.V. All rights reserved. Source

Lall P.,Auburn University | Lowe R.,Auburn University | Goebel K.,NASA
IEEE Transactions on Industrial Electronics | Year: 2012

Structural damage to ball grid array interconnects incurred during vibration testing has been monitored in the prefailure space using resistance spectroscopy-based state space vectors, rate of change of the state variable, and acceleration of the state variable. The technique is intended for condition monitoring in high reliability applications where the knowledge of impending failure is critical and the risks in terms of loss of functionality are too high to bear. Future state of the system has been estimated based on a second-order Kalman Filter model and a Bayesian Framework. The measured state variable has been related to the underlying interconnect damage in the form of inelastic strain energy density. Performance of the prognostic health management algorithm during the vibration test has been quantified using performance evaluation metrics. The methodology has been demonstrated on leadfree area-array electronic assemblies subjected to vibration. Model predictions have been correlated with experimental data. The presented approach is applicable to functional systems where corner interconnects in area-array packages may be often redundant. Prognostic metrics including α - λ precision, β accuracy, and relative accuracy have been used to assess the performance of the damage proxies. The presented approach enables the estimation of residual life based on level of risk averseness. © 2012 IEEE. Source

Brown M.E.,NASA | de Beurs K.,Virginia Polytechnic Institute and State University | Vrieling A.,University of Twente
Remote Sensing of Environment | Year: 2010

Variations in agricultural production due to rainfall and temperature fluctuations are a primary cause of food insecurity on the African continent. Analysis of changes in phenology can provide quantitative information on the effect of climate variability on growing seasons in agricultural regions. Using a robust statistical methodology, we describe the relationship between phenology metrics derived from the 26. year AVHRR NDVI record and the North Atlantic Oscillation index (NAO), the Indian Ocean Dipole (IOD), the Pacific Decadal Oscillation (PDO), and the Multivariate ENSO Index (MEI). We map the most significant positive and negative correlation for the four climate indices in Eastern, Western and Southern Africa between two phenological metrics and the climate indices. Our objective is to provide evidence of whether climate variability captured in the four indices has had a significant impact on the vegetative productivity of Africa during the past quarter century. We found that the start of season and cumulative NDVI were significantly affected by large scale variations in climate. The particular climate index and the timing showing highest correlation depended heavily on the region examined. In Western Africa the cumulative NDVI correlates with PDO in September-November. In Eastern Africa the start of the June-October season strongly correlates with PDO in March-May, while the PDO in December-February correlates with the start of the February-June season. The cumulative NDVI over this last season relates to the MEI of March-May. For Southern Africa, high correlations exist between SOS and NAO of September-November, and cumulative NDVI and MEI of March-May. The research shows that climate indices can be used to anticipate late start and variable vigor in the growing season of sensitive agricultural regions in Africa. © 2010. Source

Roberts D.A.,NASA
Journal of Geophysical Research: Space Physics | Year: 2010

Recent work has shown that at 1 AU from the Sun, the power spectrum of the solar wind magnetic field has the -5/3 spectral slope expected for Kolmogorov turbulence but that the velocity has closer to a -3/2 spectrum. This paper traces the changes in solar wind velocity spectra from 0.3 to 5 AU using data from the Helios and Ulysses spacecraft to show that this is a transient stage in solar wind evolution. The spectrum of the velocity is found to be flatter than that of the magnetic field for the higher frequencies examined for all cases until the slopes become equal (at -5/3) well past 1 AU when the wind is relatively non-Alfvénic. In some respects, in particular in the evolution of the frequency at which the spectrum changes from flatter at larger scales to a "turbulent" spectrum at smaller scales, the velocity field evolves more rapidly than the magnetic, and this is associated with the dominance of the magnetic energy over the kinetic at "inertial range" scales. The speed of the flow is argued to be largely unrelated to the spectral slopes, consistent with previous work, whereas high Alfvénicity appears to slow the spectral evolution, as expected from theory. This study shows that, for the solar wind, the idea of a simple "inertial range" with uniform spectral properties is not realistic, and new phenomenologies will be needed to capture the true situation. It is also noted that a flattening of the velocity spectrum often occurs at small scales. Source

Stott J.E.,NASA | Shtessel Y.B.,University of Alabama in Huntsville
Journal of the Franklin Institute | Year: 2012

In determining flight controls for launch vehicle systems, several uncertain factors must be taken into account, including a variety of payloads, a wide range of flight conditions and different mission profiles, wind disturbances and plant uncertainties. Crewed vehicles must adhere to human rating requirements, which limit the angular rates. Sliding mode control algorithms that are inherently robust to external disturbances and plant uncertainties are very good candidates for improving the robustness and accuracy of the flight control systems. Recently emerging Higher Order Sliding Mode (HOSM) control is even more powerful than the classical Sliding Mode Controls (SMC), including the capability to handle systems with arbitrary relative degree. This paper proposes sliding mode launch vehicle flight controls using classical SMC driven by the sliding mode disturbance observer (SMDO) and higher-order multiple and single loop designs. A case study on the SLV-X Launch Vehicle studied under a joint DARPA/Air Force program called the Force Application and Launch from CONtinental United States (FALCON) program is shown. The intensive simulations demonstrate efficacy of the proposed HOSM and SMC-SMDO control algorithms for launch vehicle attitude control. © 2011 The Franklin Institute. Published by Elsevier Ltd. All rights reserved. Source

The origin of oxidized iron in Precambrian iron formations has been debated for decades. Direct paleontological evidence for a microbial role in iron oxidation has been sought in the biosignatures in these structures. This study documents how several biosignatures of phototrophic iron-oxidizing communities form in modern hydrothermal iron deposits. The microbes, primary minerals, microfossils, and stromatolitic biofabrics from Chocolate Pots hot springs in Yellowstone National Park were characterized across a range of spatial scales via various types of microscopy, X-ray diffraction, energy dispersive spectroscopy, and total organic carbon elemental analyses. Electron microscopic examination of the cyanobacterial mats reveals the formation of distinct dendritic-like biofabrics. Early-stage iron-permineralized phototrophic microfossils display taxonomic features that allow identification to the genus level. Selected-area electron diffraction analysis indicates that cells were permineralized by iron oxides (2-line ferrihydrite). Although permineralization by silica is considered to result in fossils with the highest cellular fidelity, this investigation suggests that iron permineralization may also produce exceptionally well-preserved microfossils. Characterization of biosignatures in this modern high-iron thermal spring provides a unique opportunity to establish a link between (1) our previous physiological measurements of iron oxidation by a phototrophic community; (2) production of biosignatures by the community; and (3) survival of these biosignatures during the earliest stages of diagenesis in the iron oxides underneath the microbial mats. This fossil evidence linking taxonomy, physiology, and biosignatures may be used to infer the paleobiology and paleoecology of similar fossil benthic microbial communities and may provide a means to assess the microbial contribution to ancient iron deposits. © 2010 SEPM (Society for Sedimentary Geology). Source

Partridge J.K.,NASA
AIP Conference Proceedings | Year: 2012

The Space Shuttle uses the propellants, liquid hydrogen and liquid oxygen, to meet part of the propulsion requirements from ground to orbit. The Kennedy Space Center procured over 350 million liters of liquid hydrogen and over 200 million liters of liquid oxygen during the 30-year Space Shuttle Program. Because of the nature of the cryogenic propellants, approximately 54% of the total purchased liquid hydrogen and 32% of the total purchased liquid oxygen were used in the Space Shuttle Main Engines. The balance of the propellants were vaporized during operations for various purposes. This paper dissects the total consumption of liquid hydrogen and liquid oxygen and determines the fraction attributable to each of the various processing and launch operations that occurred during the entire Space Shuttle Program at the Kennedy Space Center. © 2012 American Institute of Physics. Source

Zenitani S.,NASA | Miyoshi T.,Hiroshima University
Physics of Plasmas | Year: 2011

Plasmoid structures in fast reconnection in low-beta plasmas are investigated by two-dimensional magnetohydrodynamic simulations. A high-resolution shock-capturing code enables us to explore a variety of shock structures: vertical slow shocks behind the plasmoid, another slow shock in the outer-region, and the shock-reflection in the front side. The Kelvin-Helmholtz-like turbulence is also found inside the plasmoid. It is concluded that these shocks are rigorous features in reconnection in low-beta plasmas, where the reconnection jet speed or the upstream Alfv́n speed exceeds the sound speed. © 2011 American Institute of Physics. Source

The combination of evolutionary with engineering principles will enhance synthetic biology. Conversely, synthetic biology has the potential to enrich evolutionary biology by explaining why some adaptive space is empty, on Earth or elsewhere. Synthetic biology, the design and construction of artificial biological systems, substitutes bio-engineering for evolution, which is seen as an obstacle. But because evolution has produced the complexity and diversity of life, it provides a proven toolkit of genetic materials and principles available to synthetic biology. Evolution operates on the population level, with the populations composed of unique individuals that are historical entities. The source of genetic novelty includes mutation, gene regulation, sex, symbiosis, and interspecies gene transfer. At a phenotypic level, variation derives from regulatory control, replication and diversification of components, compartmentalization, sexual selection and speciation, among others. Variation is limited by physical constraints such as diffusion, and chemical constraints such as reaction rates and membrane fluidity. While some of these tools of evolution are currently in use in synthetic biology, all ought to be examined for utility. A hybrid approach of synthetic biology coupled with fine-tuning through evolution is suggested. © 2010 Wiley Periodicals, Inc. Source

Oreopoulos L.,NASA | Mlawer E.,Atmospheric and Environmental Research Inc.
Bulletin of the American Meteorological Society | Year: 2010

The Continual Intercomparison of Radiation Codes (CIRC) is endorsed by the GEWEX Radiation Panel (IRC) and supported by the ARM (Energy's Atmospheric Radiation Measurement) program. CIRC uses high spectral resolution calculations as its benchmarks. It employs a ensemble ofcases in which the atmospheric and surface inputs, as well as the radiation measurements attesting to the quality of the reference calculations, are based on ARM measurements. Another feature of CIRC is an evolving and regularly updated permanent reference source that serves the global modeling community. It makes all pertinent information publicly available and is designed as a long-lasting, continual endeavor. CIRC is releasing self-contained collections of cases in stages that will be referred to as phases. The CIRC Phase I cases are drawn from the BBHRP dataset, and satisfy preset criteria that make them appropriate for the objective of this phase, which is to evaluate the RT codes under presumably the leastchallenging conditions. Source

Peslier A.H.,Jacobs Engineering | Peslier A.H.,NASA
Journal of Volcanology and Geothermal Research | Year: 2010

Olivine, pyroxene and garnet are nominally anhydrous but can accommodate tens to hundreds of parts per million (ppm) H2O or "water" in the form of protons incorporated in defects in their mineral structure. This review concerns the amount of water in nominally anhydrous minerals from mantle and mantle-derived rocks: peridotites, eclogites, megacrysts, basalts and kimberlites. Trends between internal and external parameters and water content in olivine, pyroxene, and garnet of mantle rocks allow us to identify what controls their H intake. The water content of pyroxenes and garnets in peridotites appears to depend primarily on mineral chemistry, while that of olivines may more readily reflect water activity and water fugacity conditions in the mantle. In eclogites, both mineral chemistry and metamorphic pressure control the water intake of pyroxene and garnet. The water content of minerals in crystallized melts (basalt and kimberlite phenocrysts, xenocrysts, and megacrysts) is determined by the degree of differentiation, the amount of water in the parent melt, and degassing. Basalt and cumulate minerals from Martian meteorites may be as water-rich as their Earth equivalents. No water has been detected at present in Moon minerals but low amounts in degassed basaltic glasses signify that deep Moon reservoirs may still retain water. The presence of water in mantle minerals, typically amounts of tens to hundreds of ppm, enhances their deformation properties. Water contents of peridotite minerals in the oceanic upper mantle, in and around cratons, and in subduction zones may have tremendous influence on Earth's geodynamics. © 2009 Elsevier B.V. Source

Ramsay H.A.,NASA | Sobel A.H.,Lamont Doherty Earth Observatory
Journal of Climate | Year: 2011

The effects of relative and absolute sea surface temperature (SST) on tropical cyclone potential intensity are investigated using the Massachusetts Institute of Technology (MIT) single-column model. The model is run in two modes: (i) radiative-convective equilibrium (RCE) to represent the convective response to uniform warming of the ocean as in a homogeneous aqua planet, and (ii) weak temperature gradient (WTG) to represent the convective response to warming over a limited area of ocean while the SST outside that area remains unchanged. The WTG calculations are taken to represent the sensitivity of the atmospheric state to relative SST changes, while the RCE calculations are taken to represent the sensitivity to absolute SST changes occurring in the absence of relative SST changes. The potential intensity is computed using temperature and moisture profiles from the two sets of experiments for various values of SST. The computed potential intensity is more sensitive to relative SST than to absolute SST, with slopes of between about 7 and 8 m s -1 °C -1 (depending on choice of input parameters in the model's convection scheme and other details of the model configuration) in the WTG calculations and about 1 m s -1 °C -1 in RCE. The sensitivity to relative SST obtained from these calculations is quantitatively similar to that obtained previously by G. Vecchi and B. J. Soden from global climate model output. The greater sensitivity of potential intensity to SST in the WTG simulations (relative to RCE) can be attributed primarily to larger changes in the air-sea thermodynamic disequilibrium in those calculations as SST changes, which results from the inability of the free troposphere to adjust to the SST in WTG as it does in RCE. © 2011 American Meteorological Society. Source

Barahona D.,Georgia Institute of Technology | Barahona D.,NASA | Nenes A.,Georgia Institute of Technology
Atmospheric Chemistry and Physics | Year: 2011

Low ice crystal concentration and sustained in-cloud supersaturation, commonly found in cloud observations at low temperature, challenge our understanding of cirrus formation. Heterogeneous freezing from effloresced ammonium sulfate, glassy aerosol, dust and black carbon are proposed to cause these phenomena; this requires low updrafts for cirrus characteristics to agree with observations and is at odds with the gravity wave spectrum in the upper troposphere. Background temperature fluctuations however can establish a "dynamical equilibrium" between ice production and sedimentation loss (as opposed to ice crystal formation during the first stages of cloud evolution and subsequent slow cloud decay) that explains low temperature cirrus properties. This newly-discovered state is favored at low temperatures and does not require heterogeneous nucleation to occur (the presence of ice nuclei can however facilitate its onset). Our understanding of cirrus clouds and their role in anthropogenic climate change is reshaped, as the type of dynamical forcing will set these clouds in one of two "preferred" microphysical regimes with very different susceptibility to aerosol. © 2011 Author(s). Source

Simon J.I.,NASA | Young E.D.,University of California at Los Angeles
Earth and Planetary Science Letters | Year: 2011

The difference between the precise MC-ICPMS analyses of bulk calcium-aluminum-rich inclusion (CAI) fragments (e.g., Jacobsen et al., 2008) and supra-canonical values obtained by micro-analytical techniques, e.g., laser ablation MC-ICPMS (Young et al., 2005) and SIMS (Taylor et al., 2005), at face value seems to be problematic and therefore leads many to dismiss claims of the solar system (26Al/27Al)0 greater than ~5×10-5 as spurious. Here we use mass balance calculations to quantify the importance of open system isotopic exchange during CAI evolution and show that in situ supra-canonical, in-situ canonical, and bulk canonical measurements can all exist for an individual CAI. The calculations describe mechanisms of isotopic exchange that may have occurred early (100'ska) and late (~1.5Ma) in the solar nebula and much later (>10'sMa) on parent body planetesimals. A range of possible modal mineralogies is modeled in order to populate the compositional space defined by in situ and bulk CAI measurements. In support of these simulated data we describe in situ measurements of 27Al/24Mg, 25Mg/24Mg, and 26Mg/24Mg obtained by LA-MC-ICPMS comprising core-to-rim traverses across three CV3 CAIs. The CAIs exhibit distinctive Mg isotopic zoning profiles and varying abundances of daughter products of the short-lived nuclide 26Al that are consistent with varying amounts of open system isotope exchange. © 2011. Source

Experimental studies of the partitioning of siderophile elements between metallic and silicate liquids have provided fundamental constraints on the early history and differentiation conditions of the Earth. With many new studies even in the last 20. yr, several models have emerged from the results, including low pressure equilibrium, high pressure equilibrium, and combined high and low pressure multi-stage models. The reasons - silicate melt composition, pressure effects on silicate melt structure, different methods for calculating metal activity coefficients, and the role of deep mantle phases - for the multitude of resulting models have not been specifically addressed before, yet are critical in evaluating the more likely and realistic models. The four reasons leading to the divergence of results will be discussed and evaluated. The behavior of the moderately siderophile elements Ni and Co will be compared using several approaches, each of which results in the same conclusion for Ni and Co. This consistency will eliminate the supposition that one or the other approaches gives a more accurate answer for element partitioning. Newly updated expressions for 11 elements are then derived and presented and applied to the early Earth to evaluate the idea of a late stage equilibration between a core forming metal and silicate melt (or magma ocean). It is possible to explain all 11 elements at conditions of 27-33. GPa, 3300-3600. K,ΔIW =-. 1, for peridotite and a metallic liquid containing 10% of a light element. The main difference between the current result and several other recent modeling efforts is that Mn, V, and Cr are hosted in deep mantle phases as well as the core. The other elements - Ni, Co, Mo, W, P, Cu, Ga, and Pd - are hosted in core, and detailed modeling here shows the importance of accounting for oxygen fugacity, silicate and metallic liquid compositions, as well as temperature and pressure. The idea of late stage metal-silicate equilibrium at a restricted pressure and temperature range leaving a chemical finger print on the upper mantle remains viable. © 2011. Source

Jacobson N.S.,NASA | Myers D.L.,East Central University
Journal of Physical Chemistry B | Year: 2011

The vaporization of B2O3 in a reducing environment leads to the formation of both B2O3(g) and B 2O2(g). Whereas the formation of B2O 3(g) is well understood, many questions about the formation of B 2O2(g) remain. Previous studies using B(s) + B 2O3(l) have led to inconsistent thermodynamic data. In this study, it was found that, after heating, B(s) and B2O 3(l) appeared to separate and variations in contact area likely led to the inconsistent vapor pressures of B2O2(g). To circumvent this problem, the activity of boron was fixed with a two-phase mixture of FeB and Fe2B. Both second-and third-law enthalpies of formation were measured for B2O2(g) and B 2O3(g). From these values, the enthalpies of formation at 298.15 K were calculated to be -479.9 ± 25.7 kJ/mol for B 2O2(g) and -833.4 ± 13.1 kJ/mol for B 2O3(g). Ab initio calculations to determine the enthalpies of formation of B2O2(g) and B2O3(g) were conducted using the W1BD composite method and showed good agreement with the experimental values. © 2011 American Chemical Society. Source

Gorti U.,Search for Extraterrestrial Intelligence Institute | Gorti U.,NASA | Hollenbach D.,Search for Extraterrestrial Intelligence Institute | Dullemond C.P.,University of Heidelberg
Astrophysical Journal | Year: 2015

Protoplanetary disks are dispersed by viscous evolution and photoevaporation in a few million years; in the interim small, sub-micron-sized dust grains must grow and form planets. The time-varying abundance of small grains in an evolving disk directly affects gas heating by far-ultraviolet (FUV) photons, while dust evolution affects photoevaporation by changing the disk opacity and resulting penetration of FUV photons in the disk. Photoevaporative flows, in turn, selectively carry small dust grains, leaving the larger particles - which decouple, from the gas - behind in the disk. We study these effects by investigating the evolution of a disk subject to viscosity, photoevaporation by EUV, FUV, and X-rays, dust evolution, and radial drift using a one-dimensional (1D) multi-fluid approach (gas + different dust grain sizes) to solve for the evolving surface density distributions. The 1D evolution is augmented by 1+1D models constructed at each epoch to obtain the instantaneous disk structure and determine photoevaporation rates. The implementation of a dust coagulation/fragmentation model results in a marginal decrease in disk lifetimes when compared to models with no dust evolution; the disk lifetime is thus found to be relatively insensitive to the evolving dust opacity. We find that photoevaporation can cause significant reductions in the gas/dust mass ratio in the planet-forming regions of the disk as it evolves, and may result in a corresponding increase in heavy element abundances relative to hydrogen. We discuss implications for theories of planetesimal formation and giant planet formation, including the formation of gas-poor giants. After gas disk dispersal, M of mass in solids typically remain, comparable to the solids inventory of our solar system. © 2015. The American Astronomical Society. All rights reserved. Source

Sankararaman S.,NASA
Mechanical Systems and Signal Processing | Year: 2015

This paper analyzes the significance, interpretation, and quantification of uncertainty in prognostics, with an emphasis on predicting the remaining useful life of engineering systems and components. Prognostics deals with predicting the future behavior of engineering systems, and is affected by various sources of uncertainty. In order to facilitate meaningful prognostics-based decision-making, it is important to analyze how these sources of uncertainty affect prognostics, and thereby, compute the overall uncertainty in the remaining useful life prediction. This paper investigates the classical (frequentist) and subjective (Bayesian) interpretations of uncertainty and their implications on prognostics, and argues that the Bayesian interpretation of uncertainty is more suitable for condition-based prognostics and health monitoring. It is also demonstrated that uncertainty quantification in remaining useful life prediction needs to be approached as an uncertainty propagation problem. Several uncertainty propagation methods are discussed in this context, and the practical challenges involved in such uncertainty quantification are outlined. © 2014 Elsevier Ltd. All rights reserved. Source

Ten Hoeve J.E.,Stanford University | Remer L.A.,NASA | Jacobson M.Z.,Stanford University
Atmospheric Chemistry and Physics | Year: 2011

Aerosol, cloud, water vapor, and temperature profile data from the Moderate Resolution Imaging Spectroradiometer (MODIS) are utilized to examine the impact of aerosols on clouds during the Amazonian biomass burning season in Rondnia, Brazil. It is found that increasing background column water vapor (CWV) throughout this transition season between the Amazon dry and wet seasons likely exerts a strong effect on cloud properties. As a result, proper analysis of aerosol-cloud relationships requires that data be stratified by CWV to account better for the influence of background meteorological variation. Many previous studies of aerosol-cloud interactions over Amazonia have ignored the systematic changes to meteorological factors during the transition season, leading to possible misinterpretation of their results. Cloud fraction (CF) is shown to increase or remain constant with aerosol optical depth (AOD), depending on the value of CWV, whereas the relationship between cloud optical depth (COD) and AOD is quite different. COD increases with AOD until AOD ∼ 0.3, which is assumed to be due to the first indirect (microphysical) effect. At higher values of AOD, COD is found to decrease with increasing AOD, which may be due to: (1) the inhibition of cloud development by absorbing aerosols (radiative effect/semi-direct effect) and/or (2) a possible retrieval artifact in which the measured reflectance in the visible is less than expected from a cloud top either from the darkening of clouds through the addition of carbonaceous biomass burning aerosols within or above clouds or subpixel dark surface contamination in the measured cloud reflectance. If (1) is a contributing mechanism, as we suspect, then an empirically-derived increasing function between cloud drop number and aerosol concentration, assumed in a majority of global climate models, is inaccurate since these models do not include treatment of aerosol absorption in and around clouds. The relationship between aerosols and both CWV and clouds over varying land surface types is also analyzed. The study finds that the difference in CWV between forested and deforested land is not correlated with aerosol loading, supporting the assumption that temporal variation of CWV is primarily a function of the larger-scale meteorology. However, a difference in the response of CF to increasing AOD is observed between forested and deforested land. This suggests that dissimilarities between other meteorological factors, such as atmospheric stability, may have an impact on aerosol-cloud correlations between different land cover types. © 2011 Author(s). Source

Cullather R.I.,University of Maryland College Park | Bosilovich M.G.,NASA
Journal of Climate | Year: 2011

The atmospheric moisture budget from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) is evaluated in polar regions for the period 1979-2005 and compared with previous estimates, accumulation syntheses over polar ice sheets, and in situArctic precipitation observations. The systemis based on a nonspectral background model and utilizes the incremental analysis update scheme. The annual moisture convergence from MERRA for the north polar cap is comparable to previous estimates using 40-yr European Centre for Medium-RangeWeather ForecastsRe-Analysis (ERA-40) and earlier reanalyses but it is more than 50%larger thanMERRA precipitation minus evaporation (P 2 E) computed from physics output fields. This imbalance is comparable to earlier reanalyses for the Arctic. For the south polar cap, the imbalance is 20%. The MERRA physics output fields are also found to be overly sensitive to changes in the satellite observing system, particularly over data-sparse regions of the Southern Ocean. Comparisons between MERRA and prognostic fields from two contemporary reanalyses yield a spread of values from 6% of the mean over the Antarctic Ice Sheet to 61% over a domain of the ArcticOcean. These issues highlight continued problems associated with the representation of cold-climate physical processes in global data assimilation models. The distribution of MERRA surface fluxes over the major polar ice sheets emphasizes larger values along the coastal escarpments, which agrees more closely with recent assessments of ice sheet accumulation using regional models. Differences between these results and earlier assessments illustrate a continued ambiguity in the surface moisture flux distribution over Greenland and Antarctica. The higher spatial and temporal resolution as well as the availability of all budget components, including analysis increments inMERRA, offer prospects for an improved representation of the high-latitude water cycle in reanalyses. © 2011 American Meteorological Society. Source

Taylor P.C.,NASA | Ellingson R.G.,Florida State University | Cai M.,Florida State University
Journal of Climate | Year: 2011

This study performs offline, partial radiative perturbation calculations to determine the geographical distributions of climate feedbacks contributing to the top-of-atmosphere (TOA) radiative energy budget. These radiative perturbations are diagnosed using monthly mean model output from the NCAR Community Climate System Model version 3 (CCSM3.0) forced with the Special Report Emissions Scenario (SRES) A1B emission scenario. The Monte Carlo Independent Column Approximation (MCICA) technique with a maximum-random overlap rule is used to sample monthly mean cloud frequency profiles to perform the radiative transfer calculations. It is shown that the MCICAtechnique provides a good estimate of all feedback sensitivity parameters. The radiative perturbation results are used to investigate the spatial variability of model feedbacks showing that the shortwave cloud and lapse rate feedbacks exhibit the most and second most spatial variability, respectively. It has been shown that the model surface temperature response is highly correlated with the change in the TOA net flux, and that the latter is largely determined by the total feedback spatial pattern rather than the external forcing. It is shown by representing the change in the TOA net flux as a linear combination of individual feedback radiative perturbations that the lapse rate explains the most spatial variance of the surface temperature response. Feedback spatial patterns are correlated with the model response and other feedback spatial patterns to investigate these relationships. The results indicate that the model convective response is strongly correlated with cloud and water vapor feedbacks, but the lapse rate feedback geographic distribution is strongly correlated with the climatological distribution of convection. The implication for the water vapor-lapse rate anticorrelation is discussed. © 2011 American Meteorological Society. Source

Fishman G.J.,NASA
Eos | Year: 2011

Thunderstorms are the result of warm, moist air moving rapidly upward, then cooling and condensing. Electrification occurs within thunderstorms (as noted by Benjamin Franklin), produced primarily by frictional processes among ice particles. This leads to lightning discharges; the types, intensities, and rates of these discharges vary greatly among thunderstorms. Even though scientists have been studying lightning since Franklin's time, new phenomena associated with thunderstorms are still being discovered. In particular, a recent finding by Briggs et al. [2011], based on observations by the Gamma-Ray Burst Monitor (GBM) instrument on NASA's satellite-based Fermi Gamma-ray Space Telescope (Fermi), shows that positrons are also generated by thunderstorms. Positrons are the antimatter form of electronsthey have the same mass and charge as an electron but are of positive rather than negative charge; hence the name positron. Observations of positrons from thunderstorms may lead to a new tool for understanding the electrification and high-energy processes occurring within thunderstorms. New theories, along with new observational techniques, are rapidly evolving in this field. Source

Blanchard R.C.,George Washington University | Desai P.N.,NASA
Journal of Spacecraft and Rockets | Year: 2011

The details of the trajectory and atmospheric reconstruction for the Mars Phoenix entry, descent, and landing are presented. The trajectory reconstruction used a six-degree-of-freedom process that included integrating the 200 Hz onboard inertial measurement unit incremental velocity change ΔV data and incremental angle changeΔ θ data, the derivatives from theΔV and the Δ θ data, smoothing, and correcting the inertial measurement unit accelerations for physical location on the Lander. Lander orientation angles (angle of attack and sideslip) during descent were reconstructed independently of the aerodynamics and atmosphere models. Before parachute deployment, the angle of attack and sideslip were found to be small (less than 3 deg) leading up to about 5 deg at parachute deploy, where the deployment altitude was determined to be approximately 13 km at a Mach number of about 1.7 and a dynamic pressure of 489 N=m 2. In a separate process, the atmosphere structure (i.e., density, pressure, and temperature) encountered for altitudes up to 80 km was determined. The atmosphere structure determination process involved a detailed aerodynamics model of the Lander and the parachute, as well as accounting for configuration changes during descent. Comparisons made with the preflight atmosphere model used in the mission design show good agreement with the three reconstructed atmosphere structure parameters. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. Source

Goldblatt C.,University of Victoria | Robinson T.D.,University of Washington | Zahnle K.J.,NASA | Crisp D.,Jet Propulsion Laboratory
Nature Geoscience | Year: 2013

The atmospheres of terrestrial planets are expected to be in long-term radiation balance: an increase in the absorption of solar radiation warms the surface and troposphere, which leads to a matching increase in the emission of thermal radiation. Warming a wet planet such as Earth would make the atmosphere moist and optically thick such that only thermal radiation emitted from the upper troposphere can escape to space. Hence, for a hot moist atmosphere, there is an upper limit on the thermal emission that is unrelated to surface temperature. If the solar radiation absorbed exceeds this limit, the planet will heat uncontrollably and the entire ocean will evaporate - the so-called runaway greenhouse. Here we model the solar and thermal radiative transfer in incipient and complete runaway greenhouse atmospheres at line-by-line spectral resolution using a modern spectral database. We find a thermal radiation limit of 282 W m -2 (lower than previously reported) and that 294 W m -2 of solar radiation is absorbed (higher than previously reported). Therefore, a steam atmosphere induced by such a runaway greenhouse may be a stable state for a planet receiving a similar amount of solar radiation as Earth today. Avoiding a runaway greenhouse on Earth requires that the atmosphere is subsaturated with water, and that the albedo effect of clouds exceeds their greenhouse effect. A runaway greenhouse could in theory be triggered by increased greenhouse forcing, but anthropogenic emissions are probably insufficient. Source

Pesnell W.D.,NASA
Solar Physics | Year: 2014

We describe using Ap and F10.7 as a geomagnetic-precursor pair to predict the amplitude of Solar Cycle 24. The precursor is created by using F10.7 to remove the direct solar-activity component of Ap. Four peaks are seen in the precursor function during the decline of Solar Cycle 23. A recurrence index that is generated by a local correlation of Ap is then used to determine which peak is the correct precursor. The earliest peak is the most prominent but coincides with high levels of non-recurrent solar activity associated with the intense solar activity of October and November 2003. The second and third peaks coincide with some recurrent activity on the Sun and show that a weak cycle precursor closely following a period of strong solar activity may be difficult to resolve. A fourth peak, which appears in early 2008 and has recurrent activity similar to precursors of earlier solar cycles, appears to be the "true" precursor peak for Solar Cycle 24 and predicts the smallest amplitude for Solar Cycle 24. To determine the timing of peak activity it is noted that the average time between the precursor peak and the following maximum is ≈ 6.4 years. Hence, Solar Cycle 24 would peak during 2014. Several effects contribute to the smaller prediction when compared with other geomagnetic-precursor predictions. During Solar Cycle 23 the correlation between sunspot number and F10.7 shows that F10.7 is higher than the equivalent sunspot number over most of the cycle, implying that the sunspot number underestimates the solar-activity component described by F10.7. During 2003 the correlation between aa and Ap shows that aa is 10 % higher than the value predicted from Ap, leading to an overestimate of the aa precursor for that year. However, the most important difference is the lack of recurrent activity in the first three peaks and the presence of significant recurrent activity in the fourth. While the prediction is for an amplitude of Solar Cycle 24 of 65±20 in smoothed sunspot number, a below-average amplitude for Solar Cycle 24, with maximum at 2014.5±0.5, we conclude that Solar Cycle 24 will be no stronger than average and could be much weaker than average. © 2014 The Author(s). Source

This paper solves the long-standing problem of establishing the fundamental physical link between the radiative transfer theory and macroscopic electromagnetics in the case of elastic scattering by a sparse discrete random medium. The radiative transfer equation (RTE) is derived directly from the macroscopic Maxwell equations by computing theoretically the appropriately defined so-called Poynting-Stokes tensor carrying information on both the direction, magnitude, and polarization characteristics of local electromagnetic energy flow. Our derivation from first principles shows that to compute the local Poynting vector averaged over a sufficiently long period of time, one can solve the RTE for the direction-dependent specific intensity column vector and then integrate the direction-weighted specific intensity over all directions. Furthermore, we demonstrate that the specific intensity (or specific intensity column vector) can be measured with a wellcollimated radiometer (photopolarimeter), which provides the ultimate physical justification for the use of such instruments in radiation-budget and particle-characterization applications. However, the specific intensity cannot be interpreted in phenomenological terms as signifying the amount of electromagnetic energy transported in a given direction per unit area normal to this direction per unit time per unit solid angle. Also, in the case of a densely packed scattering medium the relation of the measurement with a wellcollimated radiometer to the time-averaged local Poynting vector remains uncertain, and the theoretical modeling of this measurement is likely to require a much more complicated approach than solving an RTE. © 2010 Optical Society of America. Source

Bodson M.,University of Utah | Frost S.A.,NASA
Journal of Guidance, Control, and Dynamics | Year: 2011

Next-generation aircraft with a large number of actuators will require advanced control allocation methods to compute the actuator commands needed to follow desired trajectories while respecting system constraints. Previously, algorithms were proposed to minimize the l1 or l2 norms of the tracking error and of the actuator deflections. This paper discusses the alternative choice of the l1 norm, or the sup norm. Minimization of the control effort translates into the minimization of the maximum actuator deflection (min-max optimization). This paper shows how the problem can be solved effectively by converting it into a linear program and solving it using a simplex algorithm. Properties of the algorithm are also investigated through examples. In particular, the min-max criterion results in a type of load balancing, where the load is the desired command and the algorithm balances this load among various actuators. The solution using the l∞ norm also results in better robustness to failures and lower sensitivity to nonlinearities in illustrative examples. This paper also discusses the extension of the results to a normalized l∞ norm,\ where the norm of the actuator deflections are scaled by the actuator limits. Minimization of the control effort then\ translates into the minimization of the maximum actuator deflection as a percentage of its range of motion. Copyright © 2010. Source