Bryans Road, MD, United States
Bryans Road, MD, United States

Time filter

Source Type

Johnson B.C.,Purdue University | Lisse C.M.,JHU APL | Chen C.H.,STS Inc | Melosh H.J.,Purdue University | And 7 more authors.
Astrophysical Journal | Year: 2012

Spectral modeling of the large infrared excess in the Spitzer IRS spectra of HD 172555 suggests that there is more than 1019 kg of submicron dust in the system. Using physical arguments and constraints from observations, we rule out the possibility of the infrared excess being created by a magma ocean planet or a circumplanetary disk or torus. We show that the infrared excess is consistent with a circumstellar debris disk or torus, located at 6 AU, that was created by a planetary scale hypervelocity impact. We find that radiation pressure should remove submicron dust from the debris disk in less than one year. However, the system's mid-infrared photometric flux, dominated by submicron grains, has been stable within 4% over the last 27 years, from the Infrared Astronomical Satellite (1983) to WISE (2010). Our new spectral modeling work and calculations of the radiation pressure on fine dust in HD 172555 provide a self-consistent explanation for this apparent contradiction. We also explore the unconfirmed claim that 1047 molecules of SiO vapor are needed to explain an emission feature at 8 μm in the Spitzer IRS spectrum of HD 172555. We find that unless there are 1048 atoms or 0.05 M ⊕ of atomic Si and O vapor in the system, SiO vapor should be destroyed by photo-dissociation in less than 0.2 years. We argue that a second plausible explanation for the 8 μm feature can be emission from solid SiO, which naturally occurs in submicron silicate "smokes" created by quickly condensing vaporized silicate. © 2012. The American Astronomical Society. All rights reserved..


Lisse C.M.,JHU APL | Lisse C.M.,Johns Hopkins University | Wyatt M.C.,University of Cambridge | Chen C.H.,STS Inc | And 7 more authors.
Astrophysical Journal | Year: 2012

We have analyzed Spitzer and NASA/IRTF 2-35μm spectra of the warm, 350K circumstellar dust around the nearby MS star η Corvi (F2V, 1.4 ± 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ∼ 3AU from the central star, in the system's terrestrial habitability zone. Spectral features due to ultra-primitive cometary material were found, in addition to features due to impact produced silica and high-temperature carbonaceous phases. At least 9 × 1018 kg of 0.1-100μm warm dust is present in a collisional equilibrium distribution with dn/da ∼ a -3.5, the equivalent of a 130km radius Kuiper Belt object (KBO) of 1.0 g cm3 density and similar to recent estimates of the mass delivered to the Earth at 0.6-0.8 Gyr during the late-heavy bombardment. We conclude that the parent body was a Kuiper Belt body or bodies which captured a large amount of early primitive material in the first megayears of the system's lifetime and preserved it in deep freeze at ∼150AU. At ∼1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10kms-1 with a rocky planetary body of mass ≤M Earth at ∼3AU, delivering large amounts of water (>0.1% of M Earth 's Oceans) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO. © 2012. The American Astronomical Society. All rights reserved.


Lui A.T.Y.,JHU APL | Zong Q.-G.,Beijing Institute of Technology | Zong Q.-G.,University of Massachusetts Lowell | Wang C.,Chinese Academy of Sciences | Dunlop M.W.,Rutherford Appleton Laboratory
Journal of Geophysical Research: Space Physics | Year: 2012

We examine the strength of the electron source associated with dipolarization at the outer boundary of the radiation belts using multisatellite observations from THEMIS. This topic is relevant to the determination on the relative roles of inward radial diffusion versus internal local acceleration for the origin of the relativistic electrons in the outer radiation belt. We focus on the electron phase space density (PSD) as a function of the first adiabatic invariant () for equatorially mirroring population over a broad energy range. It is found that the source strength associated with dipolarization for non-storm periods at the outer boundary of the radiation belts can be well above the observed fluxes of relativistic electrons inside the outer radiation belt. The PSD change due to the magnetic field strength variation dominates over PSD change from the energy flux increase with dipolarization, resulting in a strong anticorrelation between magnetic field strength and PSD values at a given . If observations from closely spaced satellites during the same event can be used to indicate radial transport of electrons with dipolarization, then the observed PSD at these satellites indicates frequent occurrence of non-adiabatic process during their radial transport. © 2012. American Geophysical Union. All Rights Reserved.


Connors M.,Athabasca University | McPherron R.L.,University of California at Los Angeles | Anderson B.J.,JHU APL | Korth H.,JHU APL | And 2 more authors.
Geophysical Research Letters | Year: 2014

The three-dimensional "substorm current wedge" (SCW) was postulated by McPherron et al. (1973) to explain substorm magnetic perturbations. The origin and coherence as a physical system of this important paradigm of modern space physics remained unclear, however, with progress hindered by gross undersampling, and uniqueness problems in data inversion. Complementing AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) space-derived radial electric currents with ground magnetic data allowing us to determine currents from the ionosphere up, we overcome problems of uniqueness identified by Fukushima (1969, 1994). For a substorm on 24 February 2010, we quantify SCW development consistently from ground and space data. Its westward electrojet carries 0.5 MA in the more poleward part of the auroral oval, in Region 1 (R1) sense spanning midnight. The evening sector electrojet also feeds into its upward current. We thus validate the SCW concept and obtain parameters needed for quantitative study of substorms. Key Points Auroral zone electric currents should be determined from both ground and space The substorm current wedge to first order represents substorm currents well Magnetic inversions can give useful quantitative parameters of the current wedge ©2014. American Geophysical Union. All Rights Reserved.


Lisse C.M.,JHU APL | Sitko M.L.,University of Cincinnati | Sitko M.L.,Space Science Institute | Marengo M.,Iowa State University
Astrophysical Journal Letters | Year: 2015

We have utilized the NASA/IRTF 3 m SpeX instrument's high-resolution spectral mode to observe and characterize the near-infrared flux emanating from the unusual Kepler light curve system KIC 8462852. By comparing the resulting 0.8-4.2 μm spectrum to a mesh of model photospheric spectra, the 6 emission line analyses of the Rayner et al. catalog, and the 25 system collections of debris disks we have observed to date using SpeX under the Near InfraRed Debris disk Survey, we have been able to additionally characterize the system. Within the errors of our measurements, this star looks like a normal solar abundance main-sequence F1V to F3V dwarf star without any obvious traces of significant circumstellar dust or gas. Using Connelley & Greene's emission measures, we also see no evidence of significant ongoing accretion onto the star nor any stellar outflow away from it. Our results are inconsistent with large amounts of static close-in obscuring material or the unusual behavior of a YSO system, but are consistent with the favored episodic giant comet models of a Gyr old stellar system favored by Boyajian et al. We speculate that KIC 8462852, like the ∼1.4 Gyr old F2V system η Corvi, is undergoing a late heavy bombardment, but is only in its very early stages. © 2015. The American Astronomical Society. All rights reserved.


Grant
Agency: Department of Defense | Branch: Missile Defense Agency | Program: STTR | Phase: Phase II | Award Amount: 877.97K | Year: 2014

Corvid Technologies is pleased to offer this STTR Phase II proposal in collaboration with The Johns Hopkins University Applied Physics Laboratory (JHU/APL), Spectral Sciences Inc. and Torch Technologies. Capabilities from each collaborator are being combined toward the ability to perform accurate, fast-running electro-optical and infrared (EO/IR) signature predictions. This collaborative effort will focus on the realization of consolidated modeling capabilities and benchmarking against empirical data. When coupled with existing Corvid radar modeling efforts, the resulting capability will provide a unified description across disparate sensor domains including radar and EO/IR. Approved for Public Release 14-MDA-7739 (18 March 14).


The Radiation Belt Storm Probe (RBSP) spacecraft used the Direct Field Acoustic Testing (DFAT) method to meet the program's acoustic test requirement. Leading up to the flight test, a representative test article (Mockup) was tested by the direct field acoustic method and in a reverberant chamber. The Mockup was designed to be a pathfinder for RBSP spacecraft by mimicking the overall shape and construction of the flight satellites. Microphone placement relative to the Mockup and accelerometer locations remained consistent between the tests allowing for detailed comparisons. Mockup accelerometer responses to a reverberant field are compared to accelerometer responses generated from a direct acoustic field using MIMO control. In addition, microphones 9" from the panel surfaces are compared between reverberant and DFAT tests. Flight RBSP accelerometer responses are presented for selected areas.


Lui A.T.Y.,JHU APL
Geophysical Research Letters | Year: 2016

The physical process responsible for the onset of substorm expansion is still unresolved in spite of decades of research on the topic. Detailed properties of the spatially periodic auroral beads on prebreakup auroral arcs that initiate substorm expansion onset are now available. These auroral bead properties impose severe observational constraints on the onset process. In this work, theoretical predictions of the cross-field current instability are evaluated in terms of these constraints. The growth rates and wavelengths associated with auroral beads in several previously published events are reproduced by the cross-field current instability, implying that the instability can indeed account for the characteristics of auroral beads that eventually lead to substorm onset. The present results differ from the conclusion reached by a previous analysis that the shear flow ballooning instability can account for the growth and spatial scales of auroral beads better than the cross-field current instability. ©2016. American Geophysical Union. All Rights Reserved.


Lui A.T.Y.,JHU APL
Geophysical Research Letters | Year: 2016

The modification of current density on the dawn-dusk cross section of the magnetotail with the earthward approach of a dipolarization front (DF) is examined through the recently published results of a three-dimensional (3-D) particle-in-cell (PIC) simulation. It is found that the current density intensifies by ~37% abruptly within ~1.5 ion gyrotime as the DF approaches and shows localized regions with north-south extrusions. After reaching its peak value, it undergoes a drastic current reduction (DCR) by ~65% within ~2 ion gyrotime. Breakdown of the frozen-in condition occurs in the neutral sheet region in association with DCR, demonstrating the non-MHD behavior of the phenomenon. The evolution of current density from this 3-D PIC simulation bears several similarities to those observed for the current disruption (CD) phenomenon, such as explosive growth and disruption of the current density leading to a breakdown of the frozen-in condition. The evolution is also similar to those from a previous two-dimensional (2-D) PIC simulation specially designed to investigate the nonlinear evolution of the cross-field current instability for CD. One interpretation of these findings is that CD and substorm triggering can be associated with earthward intrusion of a DF into the near-Earth plasma sheet as indicated by previous Cluster and Time History of Events and Macroscale Interactions during Substorms observations. An alternative interpretation is that both DF and CD are consequences of a global evolution from an ion-tearing-like instability of the magnetotail. ©2016. American Geophysical Union. All Rights Reserved.


Lui A.T.Y.,JHU APL
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2014

There is a long history in the study of current-disruptions/dipolarizations (CDDs) in the Earth's magnetotail. A recent trend for this topic is the focus on very transient (~1-2min) positive pulses of the Bz component in the magnetotail coined as dipolarization fronts (DFs). We reviewed several salient features of CDDs reported previously and compare them with those of DFs. We find several major differences between them. First, their temporal profiles differ significantly-DFs have pulse-like form while CDDs have the sustained dipolarization lasting for many minutes. Second, CDDs are typically associated with initial large magnetic fluctuations having the characteristics of turbulence while DFs are not. Third, DFs typically propagate Earthward while CDDs spread tailward. Fourth, DFs are mainly spatial structures acting as discontinuities to separate the regions ahead and behind them with a north-south oriented thin current sheet. On the other hand, CDDs are mainly temporal manifestations of a local dynamic process that reduces the east-west cross-tail current near the neutral sheet. There is indication that CDDs in the near-Earth region (within the downstream distance of ~15RE) occur prior to substorm onset and DFs are typically found in the midtail region after substorm onsets. These differences justify renaming DFs as dipolarization pulses so that the fronts of CDDs can be distinguished from dipolarization pulses without the confusion brought about by the present terminology. © 2013 Elsevier Ltd.

Loading JHU APL collaborators
Loading JHU APL collaborators