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Leka K.D.,NWRA | Barnes G.,NWRA | Wagner E.L.,NWRA
Solar Physics | Year: 2017

Although for many solar physics problems the desirable or meaningful boundary is the radial component of the magnetic field Br, the most readily available measurement is the component of the magnetic field along the line of sight to the observer, Blos. As this component is only equal to the radial component where the viewing angle is exactly zero, some approximation is required to estimate Br at all other observed locations. In this study, a common approximation known as the “μ-correction”, which assumes all photospheric field to be radial, is compared to a method that invokes computing a potential field that matches the observed Blos, from which the potential field radial component, Brpot is recovered. We demonstrate that in regions that are truly dominated by a radially oriented field at the resolution of the data employed, the μ-correction performs acceptably if not better than the potential-field approach. However, it is also shown that for any solar structure that includes horizontal fields, i.e. active regions, the potential-field method better recovers both the strength of the radial field and the location of magnetic neutral line. © 2017, Springer Science+Business Media Dordrecht.

Centeno R.,High Altitude Observatory | Schou J.,Stanford University | Schou J.,Max Planck Institute for Solar System Research | Hayashi K.,Stanford University | And 5 more authors.
Solar Physics | Year: 2014

The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington spectral line inversion code used to determine the magnetic and thermodynamic parameters of the solar photosphere from observations of the Stokes vector in the 6173 Å Fe i line by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We report on the modifications made to the original VFISV inversion code in order to optimize its operation within the HMI data pipeline and provide the smoothest solution in active regions. The changes either sped up the computation or reduced the frequency with which the algorithm failed to converge to a satisfactory solution. Additionally, coding bugs which were detected and fixed in the original VFISV release are reported here. © 2014 Springer Science+Business Media Dordrecht.

Birch A.C.,NWRA | Braun D.C.,NWRA | Fan Y.,High Altitude Observatory
Astrophysical Journal Letters | Year: 2010

The physics of the formation of magnetic active regions (ARs) is one of the most important problems in solar physics. One main class of theories suggests that ARs are the result of magnetic flux that rises from the tachocline. Time-distance helioseismology, which is based on measurements of wave propagation, promises to allow the study of the subsurface behavior of this magnetic flux. Here, we use a model for a buoyant magnetic flux concentration together with the ray approximation to show that the dominant effect on the wave propagation is expected to be from the roughly 100 m s-1 retrograde flow associated with the rising flux. Using a B-spline-based method for carrying out inversions of wave travel times for flows in spherical geometry, we show that at 3 days before emergence the detection of this retrograde flow at a depth of 30Mm should be possible with a signal-to-noise level of about 8 with a sample of 150 emerging ARs. © 2010 The American Astronomical Society. All rights reserved.

Evan S.,University of Colorado at Boulder | Alexander M.J.,NWRA | Dudhia J.,NCAR
Journal of the Atmospheric Sciences | Year: 2012

A 2-day inertia-gravity wave (IGW) was observed in high-resolution radiosonde soundings of horizontal wind and temperature taken during the 2006 Tropical Warm Pool-International Cloud Experiment (TWPICE) experiment in the Darwin area. The wave was observed in the stratosphere above Darwin from 28 January to 5 February.Asimilar wave event is observed in the European Centre for Medium-Range Weather Forecasts (ECMWF) operational data. A comparison between the characteristics of the IGW derived with the ECMWF data to the properties of the wave derived with the radiosonde data shows that the ECMWFdata capture similar structure for this 2-day wave event but with a larger vertical wavelength. A reverse ray-tracing method is used to localize the source region. Using ECMWF data to define the atmospheric background conditions and wave properties observed in the soundings, it is found that the 2-day wave event originated from deep convection in the Indonesian region around 20 January. The Weather Research and Forecasting (WRF) modeling system is used to complement the ECMWF data to assess the influence of vertical resolution and initial conditions on the wave structure. The model domain is configured as a tropical channel and the ECMWF analyses provide the north/south boundaries and initial conditions. WRF is used with the same horizontal resolution (40 km) as the operational ECMWF in 2006 while using a finer vertical grid spacing than ECMWF. The model is run from 18 January to 11 February to cover the wave life cycle. Different experiments are also performed to determine the sensitivity of the wave structure to cumulus schemes, initial conditions, and vertical resolution. The 2-day wave properties resulting from the WRF experiments are compared to those retrieved from the radiosonde data and from the ECMWF analyses. It is demonstrated that higher vertical resolution would be required for ECMWF to accurately resolve the vertical structure of the wave and its effect on the middle-atmospheric circulation. © 2012 American Meteorological Society.

Milliff R.F.,NWRA | Bonazzi A.,Operational Oceanography Group | Wikle C.K.,University of Missouri | Pinardi N.,Operational Oceanography Group | Berliner L.M.,Ohio State University
Quarterly Journal of the Royal Meteorological Society | Year: 2011

A Bayesian hierarchical model (BHM) is developed to estimate surface vector wind (SVW) fields and associated uncertainties over the Mediterranean Sea. The BHM-SVW incorporates data-stage inputs from analyses and forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) and SVW retrievals from the QuikSCAT data record. The process-model stage of the BHM-SVW is based on a Rayleigh friction equation model for surface winds. Dynamical interpretations of posterior distributions of the BHM-SVW parameters are discussed. Ten realizations from the posterior distribution of the BHM-SVW are used to force the data-assimilation step of an experimental ensemble ocean forecast system for the Mediterranean Sea in order to create a set of ensemble initial conditions. The sequential data-assimilation method of the Mediterranean forecast system (MFS) is adapted to the ensemble implementation. Analyses of sample ensemble initial conditions for a single data-assimilation period in MFS are presented to demonstrate the multivariate impact of the BHM-SVW ensemble generation methodology. Ensemble initial-condition spread is quantified by computing standard deviations of ocean state variable fields over the ten ensemble members. The methodological findings in this article are of two kinds. From the perspective of statistical modelling, the process-model development is more closely related to physical balances than in previous work with models for the SVW. From the ocean forecast perspective, the generation of ocean ensemble initial conditions via BHM is shown to be practical for operational implementation in an ensemble ocean forecast system. Phenomenologically, ensemble spread generated via BHM-SVW occurs on ocean mesoscale time- and space-scales, in close association with strong synoptic-scale wind-forcing events. A companion article describes the impacts of the BHM-SVW ensemble method on the ocean forecast in comparisons with more traditional ensemble methods. Copyright © 2011 Royal Meteorological Society.

Nicolls M.J.,SRI International | Vadas S.L.,NWRA | Meriwether J.W.,Clemson University | Conde M.G.,University of Alaska Fairbanks | Hampton D.,University of Alaska Fairbanks
Journal of Geophysical Research: Space Physics | Year: 2012

In a companion paper, we derived the high-frequency, compressible, dissipative polarization relations for gravity waves (GWs) propagating in the thermosphere. In this paper, we apply the results to nighttime thermospheric observations of a GW over Alaska on 9-10 January 2010. Using a vertically-pointed Fabry-Perot interferometer (FPI) at Poker Flat that measured vertical wind perturbations (w') and two FPIs that measured the line-of-sight (LOS) velocities in four common volumes, we inferred a GW ground-based period ∼32.7 ± 0.3 min, horizontal wavelength λH=1094 ± 408 km, horizontal ground-based phase speed cH∼560 ± 210m/s, and propagation azimuth θ∼33.5 ± 15.8° east-of-north. We compared the phase shifts and amplitude ratios of this GW with that predicted by the GW dissipative polarization relations derived in the companion paper, enabled by the ability of the FPIs to measure fundamental GW parameters (wind and temperature perturbations). We find that GWs with λH∼700-1100 km, λz∼-500 to -350 km, θ∼ 15 to 50, and cH∼350-560m/s agree with the observations if the primary contribution to the 630-nm emission was near the upper portion of that layer. The source of GW was likely thermospheric given the large intrinsic phase speed of the wave. Possible sources are discussed, the most likely of which are related to the onset of auroral activity near the time that the wave was initially observed. Copyright 2012 by the American Geophysical Union.

We use a publicly available numerical wave-propagation simulation of Hartlep et al. (Solar Phys. 268, 321, 2011) to test the ability of helioseismic holography to detect signatures of a compact, fully submerged, 5 % sound-speed perturbation placed at a depth of 50 Mm within a solar model. We find that helioseismic holography employed in a nominal "lateral-vantage" or "deep-focus" geometry employing quadrants of an annular pupil can detect and characterize the perturbation. A number of tests of the methodology, including the use of a plane-parallel approximation, the definition of travel-time shifts, the use of different phase-speed filters, and changes to the pupils, are also performed. It is found that travel-time shifts made using Gabor-wavelet fitting are essentially identical to those derived from the phase of the Fourier transform of the cross-covariance functions. The errors in travel-time shifts caused by the plane-parallel approximation can be minimized to less than a second for the depths and fields of view considered here. Based on the measured strength of the mean travel-time signal of the perturbation, no substantial improvement in sensitivity is produced by varying the analysis procedure from the nominal methodology in conformance with expectations. The measured travel-time shifts are essentially unchanged by varying the profile of the phase-speed filter or omitting the filter entirely. The method remains maximally sensitive when applied with pupils that are wide quadrants, as opposed to narrower quadrants or with pupils composed of smaller arcs. We discuss the significance of these results for the recent controversy regarding suspected pre-emergence signatures of active regions. © 2012 Springer Science+Business Media Dordrecht.

Birch A.C.,NWRA
Journal of Physics: Conference Series | Year: 2011

Local helioseismology is a set of methods that are used to study wave propagation and infer physical conditions in the solar interior. Sunspots are a particularly challenging target for local helioseismology. In this review, I will show that some new methods (magnetoconvection simulations and numerical wave propagation simulations) lead to shallow sunspot models that are apparently inconsistent with traditional inferences from local helioseismology. In addition, I will show that inferences for the depth structure of moat flows are not in general agreement either. © Published under licence by IOP Publishing Ltd.

Braun D.C.,NWRA | Wan K.,NWRA
Journal of Physics: Conference Series | Year: 2011

A variety of local-helioseismic analyses have shown ∼50 m/s flows converging on active regions (ARs). We have examined the average properties of both the 75 strongest converging and 75 strongest diverging flows present in Carrington rotation CR1988 within the uppermost 3 Mm of the Sun. The flows, averaged over 5 days, were deduced from calibrated helioseismic holography measurements applied to MDI observations of CR1988. Inflows associated with ARs typically have maximum speed of between 20 and 60 m/s at about 3 heliocentric degrees from their centers and fall to zero by a radius of 7 degrees. Similar converging flows, however, are prevalent in the quiet Sun. Outflows of similar spatial extent, but signficantly larger speeds, are present diverging from sunspots (i.e. the moat flows). Many of the converging flows in ARs appear to simply mark the boundaries of the moats while others converge on plage regions. In general, large ARs containing sunspots contain a complex mixture of both inflows and outflows which, aside from sunspot moats, also appear similar in property to convective components of the quiet Sun. © Published under licence by IOP Publishing Ltd.

We derive the analytic, linear, f-plane compressible solutions to local, interval, 3-D horizontal and vertical body forces, and heat/coolings in an isothermal, unsheared, and nondissipative atmosphere. These force/heat/coolings oscillate at the frequency, and turn on and off smoothly over a finite interval in time. The solutions include a mean response, gravity waves (GWs), and acoustic waves (AWs). The excited waves span a large range of horizontal/vertical scales and frequencies ω. We find that the compressible solutions are important for GWs with vertical wavelengths |λz|>(1to2)×πH if the depth of the force/heat/cooling is greater than the density scale height H. We calculate the primary GWs excited by a deep convective plume, ray trace them into the thermosphere, and calculate the body force/heat/coolings which result where the GWs dissipate. We find that the force/heat/cooling amplitudes are up to ∼40% smaller using the compressible (as compared to the Boussinesq) GW spectra. For a typical plume, the force/heat/coolings are deeper than H and have maximum amplitudes of ∼0.2 to 0.6 m/s2and ∼0.06 to 0.15 K/s for solar maximum to minimum, respectively. The heat/cooling consists of dipoles at z∼150-200 km and a heating at z∼240-260 km. We find that the compressible solutions are necessary for calculating the secondary GWs excited by these thermospheric force/heat/coolings. ©2013. American Geophysical Union. All Rights Reserved.

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