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NASA NTRS1986

Variability studies of 3C 371

The compact extragalactic radio source, 3C 371, was observed with the X-ray detectors of the EXOSAT satellite for 19.5 hours in 2 observing periods separated by 18 days. This resulted in the discovery of X-ray variability of the source on time scales between 8 hours and about 25 minutes and the confirmation of earlier reports of variability on longer time scales. The short time scale variability agrees remarkably well with earlier predictions based on fitting multifrequency data from the source to a relativistically beamed inhomogeneous synchrotron self-Compton (SSC) jet model. This SSC model is frequently applied to BL Lac objects and related sources, such as 3C 371. However, the number of free parameters in model fits tends to be large, and so independent support such as from this work is important. The X-ray spectral results for 3C 371 also may provide qualitative support for the SSC model in that the spectrum probably consists of two components, with the steeper one at low photon energies. In terms of the model, the low-energy spectrum would be dominated by synchrotron emission extending down to radio energies, whereas the higher energy X-rays would be dominated by Compton radiation.

Worrall, D. M.↗

NASA NTRS1994

Reanalysis of X-ray emission from M87. 2: The multiphase medium

In a previous paper, we showed that a single-phase model for the gas around M87 simultaneously explained most available X-ray data. Total enclosed masses derived from the model, however, fell well below the determinations from optical measurements. In this paper, we consider possible solutions to the inconsistency, including two multiphase medium models for the gas and the consequences of systematic errors of the Einstein Focal Point Crystal Spectrometer (FPCS). First, we find that when constraints from optical mass determinations are not considered, the best-fit model to the X-ray data is always the single-phase model. Multiphase models or consideration of FPCS systematic errors are required only when optical mass constraints are included. We find that the cooling time model of White & Sarazin adequately explains the available X-ray data and predicts total masses which agree with optical measurements. An ad hoc power-law multiphase does not. This shows both that the existence of mass dropping out of the ambient phase is consistent with the data and that the cooling-time model gives a reasonable parameterization of the dropout rate. Our derived mass accretion rate is similar to previous determinations. The implications of this result for cluster mass determinations in general are discussed. We then consider 'self absorbing' models where we assume that material dropping out of the ambient medium goes completely into X-ray absorbing gas. The resulting internal absorption is small compared to Galactic absorption at most radii. The models are therefore indistinguishable from models with only Galactic absorption. We finally show that it is alternatively possible to simultaneously fit optical mass measurements and X-ray data with a single-phase model if some of the observed FPCS line fluxes are too high by the maximum systematic error. This possiblity can be checked with new data from satellites such as ASCA.

Tsai, John C.↗

NASA NTRS1993

The abundances of hydrogen, helium, oxygen, and iron accelerated in large solar particle events

Energy spectra measured in 10 large flares with the University of Maryland/Max-Planck-Institut sensors on ISEE I and Goddard Space Flight Center sensors on IMP 8 allowed us to determine the average H, He, O, and Fe abundances as functions of energy in the range of about 0.3-80 MeV/nucleon. Model fits to the spectra of individual events using the predictions of a steady state stochastic acceleration model with rigidity-dependent diffusion provided a means of interpolating small portions of the energy spectra not measured with the instrumentation. Particles with larger mass-to-charge ratios were relatively less abundant at higher energies in the flare-averaged composition. The Fe/O enhancement at low SEP energies was less than the Fe/O ratios observed in He-3-rich flares. Unlike the SEP composition averaged above 5 MeV/nucleon, the average SEP abundances above 0.3 MeV/nucleon were similar to the average solar wind.

Mazur, J. E.↗

NASA NTRS2005

Regression Analysis of Long-Term Profile Ozone Data Set from BUV Instruments

We have produced a profile merged ozone data set (MOD) based on the SBUV/SBUV2 series of nadir-viewing satellite backscatter instruments, covering the period from November 1978 - December 2003. In 2004, data from the Nimbus 7 SBUV and NOAA 9, ll, and 16 SBUV/2 instruments were reprocessed using the Version 8 (V8) algorithm and most recent calibrations. More recently, data from the Nimbus 4 BUT instrument, which was operational from 1970 - 1977, were also reprocessed using the V8 algorithm. As part of the V8 profile calibration, the Nimbus 7 and NOAA 9 (1993-1997 only) instrument calibrations have been adjusted to match the NOAA 11 calibration, which was established based on comparisons with SSBUV shuttle flight data. Differences between NOAA 11, Nimbus 7 and NOAA 9 profile zonal means are within plus or minus 5% at all levels when averaged over the respective periods of data overlap. NOAA 16 SBUV/2 data have insufficient overlap with NOAA 11, so its calibration is based on pre-flight information. Mean differences over 4 months of overlap are within plus or minus 7%. Given the level of agreement between the data sets, we simply average the ozone values during periods of instrument overlap to produce the MOD profile data set. Initial comparisons of coincident matches of N4 BUV and Arosa Umkehr data show mean differences of 0.5 (0.5)% at 30km; 7.5 (0.5)% at 35 km; and 11 (0.7)% at 40 km, where the number in parentheses is the standard error of the mean. In this study, we use the MOD profile data set (1978-2003) to estimate the change in profile ozone due to changing stratospheric chlorine levels. We use a standard linear regression model with proxies for the seasonal cycle, solar cycle, QBO, and ozone trend. To account for the non-linearity of stratospheric chlorine levels since the late 1990s, we use a time series of Effective Chlorine, defined as the global average of Chlorine + 50 * Bromine at 1 hPa, as the trend proxy. The Effective Chlorine data are taken from the 3-D Goddard CTM. We will show the latest trend results using this statistical model. In addition, the Nimbus 4 BUV data offer an opportunity to test the physical properties of our statistical model. From ground-based comparisons we will establish an uncertainty range for the Nimbus 4 data. We then extrapolate our statistical model fit backwards in time and compare to the Nimbus 4 data. We compare the characteristics of the residual, defined as the difference between the data and statistical regression fit, during the Nimbus 4 time period and the 1978-2003 period over which the statistical model coefficients were estimated, and present these results.

Stolarski, Richard S.↗

NASA NTRS2011

Fermi/GBM Observations of SGRJ0501 + 4516 Bursts

We present our temporal and spectral analyses of 29 bursts from SGRJ0501+4516, detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope during the 13 days of the source activation in 2008 (August 22 to September 3). We find that the T(sub 90) durations of the bursts can be fit with a log-normal distribution with a mean value of approx. 123 ms. We also estimate for the first time event durations of Soft Gamma Repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T(sub 90)s estimated in count space (following a log-normal distribution with a mean value of approx. 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two black body functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that E(sub peak) decreases with energy flux (and fluence) to a minimum of approx. 30 keV at F = 8.7 x 10(exp -6)erg/sq cm/s, increasing steadily afterwards. Two more sources exhibit a similar trend: SGRs J1550 - 5418 and 1806 - 20. The isotropic luminosity, L(sub iso), corresponding to these flux values is roughly similar for all sources (0.4 - l.5 x 10(exp 40) erg/s.

Lin, Lin↗

NASA NTRS1981

Numerical model of the moon's rotation

The differential equations for the Euler angles that define the moon's orientation with respect to an inertial coordinate frame have been integrated numerically. This numerical model of the moon's rotation, when used in conjunction with current lunar-orbit and earth-rotation models, fits lunar laser ranging observations over a five-year period within 28 cm (rms). The present model is also compared with the numerical model of Williams (1975) and Eckhardt's (1981) semianalytical model.

Cappalo, R. J.↗

NASA NTRS1986

Estimation of canopy parameters for inhomogeneous vegetation canopies from reflectance data. II - Estimation of leaf area index and percentage of ground cover for row canopies

The canopy reflectance (CR) model for row-planted vegetation proposed earlier has been tested for soybean canopies in three different stages of growth and for corn canopies at early and full growth stages. The model fits the field-measured bidirectional CR data quite well. It is shown that, by inverting this model, one could estimate the leaf area index as well as the percentage of ground cover quite accurately from measured canopy reflectances.

Goel, N. S.↗

NASA NTRS2004

Statistical analysis of geodetic networks for detecting regional events

We present an application of hidden Markov models (HMMs) to analysis of geodetic time series in Southern California. Our model fitting method uses a regularized version of the deterministic annealing expectation-maximization algorithm to ensure that model solutions are both robust and of high quality.

geodetic networks↗

NASA NTRS2021

A Physical Model for a Slim Convective Dusty Disk in AGN

An accretion disk in an Active Galactic Nucleus (AGN) harbors and shields dust from external illumination. Our model shows that at the mid-plane of the disk around a MBH = 107M black hole, dust can exist at 0:1pc from the black hole, compared to 0.5pc outside of the disk where such self-shielding is constrained. We construct a physical model of a disk region approximately located between the radius of dust sublimation at the disk mid-plane and the radius at which dust sublimes at the disk surface. Our main conclusion is that for a wide range of model parameters such as local accretion rate and/or opacity, the accretion disk’s own radiation pressure on dust significantly influences its vertical structure. In this region convection plays important role in the vertical transport of energy. When the local accretion rate exceeds 2:5M yr the 10􀀀2pc-scale disk is super-critical with respect to dust opacity. Such a disk puffs up and transforms from geometrically thin to slim. Our model fits into the narrative of a "failed wind" scenario of Czerny & Hryniewicz (2011) and the "compact torus" model of Baskin & Laor (2018), incorporating them as variations of the radiative dusty disk model.

A Dorodnitsyn↗

NASA NTRS2018

Secondary eclipses of WASP-18b – near-infrared observations with the Anglo-Australian Telescope, the Magellan Clay Telescope and the LCOGT network

We present new eclipse observations for one of the hottest ‘hot Jupiters', WASP-18b, for which previously published data from HST WFC3 and Spitzer have led to radically conflicting conclusions about the composition of this planet’s atmosphere. We measure eclipse depths of 0.15 ± 0.02 per cent at Ks and 0.07 ± 0.01 per cent at z′ bands. Using the VSTAR line-by-line radiative transfer code and both these new observations with previously published data, we derive a new model of the planetary atmosphere. We have varied both the metallicity and C/O ratio in our modelling, and find no need for the extreme metallicity suggested by Sheppard et al. Our best-fitting models slightly underestimate the emission at z′ band and overestimate the observed flux at Ks-band. To explain these discrepancies, we examine the impact on the planetary emission spectrum of the presence of several types of hazes which could form on the night side of the planet. Our Ks-band eclipse flux measurement is lower than expected from clear atmosphere models and this could be explained by haze particles larger than 0.2 μm with the optical properties of Al2O3, CaTiO3 or MgSiO3. We find that z′-band measurements are important for understanding the contribution of photochemical hazes with particles smaller than 0.1 μm at the top of the atmosphere.

L. Kedziora-Chudczer↗

NASA NTRS2010

Blowout Jets: Hinode X-Ray Jets that Don't Fit the Standard Model

Nearly half of all H-alpha macrospicules in polar coronal holes appear to be miniature filament eruptions. This suggests that there is a large class of X-ray jets in which the jet-base magnetic arcade undergoes a blowout eruption as in a CME, instead of remaining static as in most solar X-ray jets, the standard jets that fit the model advocated by Shibata. Along with a cartoon depicting the standard model, we present a cartoon depicting the signatures expected of blowout jets in coronal X-ray images. From Hinode/XRT movies and STEREO/EUVI snapshots in polar coronal holes, we present examples of (1) X-ray jets that fit the standard model, and (2) X-ray jets that do not fit the standard model but do have features appropriate for blowout jets. These features are (1) a flare arcade inside the jet-base arcade in addition to the small flare arcade (bright point) outside that standard jets have, (2) a filament of cool (T is approximately 80,000K) plasma that erupts from the core of the jetbase arcade, and (3) an extra jet strand that should not be made by the reconnection for standard jets but could be made by reconnection between the ambient unipolar open field and the opposite-polarity leg of the filament-carrying flux-rope core field of the erupting jet-base arcade. We therefore infer that these non-standard jets are blowout jets, jets made by miniature versions of the sheared-core-arcade eruptions that make CMEs

Moore, Ronald L.↗

NASA NTRS1990

On the Response of the Special Sensor Microwave/Imager to the Marine Environment: Implications for Atmospheric Parameter Retrievals

A reasonably rigorous basis for understanding and extracting the physical information content of Special Sensor Microwave/Imager (SSM/I) satellite images of the marine environment is provided. To this end, a comprehensive algebraic parameterization is developed for the response of the SSM/I to a set of nine atmospheric and ocean surface parameters. The brightness temperature model includes a closed-form approximation to microwave radiative transfer in a non-scattering atmosphere and fitted models for surface emission and scattering based on geometric optics calculations for the roughened sea surface. The combined model is empirically tuned using suitable sets of SSM/I data and coincident surface observations. The brightness temperature model is then used to examine the sensitivity of the SSM/I to realistic variations in the scene being observed and to evaluate the theoretical maximum precision of global SSM/I retrievals of integrated water vapor, integrated cloud liquid water, and surface wind speed. A general minimum-variance method for optimally retrieving geophysical parameters from multichannel brightness temperature measurements is outlined, and several global statistical constraints of the type required by this method are computed. Finally, a unified set of efficient statistical and semi-physical algorithms is presented for obtaining fields of surface wind speed, integrated water vapor, cloud liquid water, and precipitation from SSM/I brightness temperature data. Features include: a semi-physical method for retrieving integrated cloud liquid water at 15 km resolution and with rms errors as small as approximately 0.02 kg/sq m; a 3-channel statistical algorithm for integrated water vapor which was constructed so as to have improved linear response to water vapor and reduced sensitivity to precipitation; and two complementary indices of precipitation activity (based on 37 GHz attenuation and 85 GHz scattering, respectively), each of which are relatively insensitive to variations in other environmental parameters.

Petty, Grant W.↗

NASA NTRS2002

Transport and Growth Kinetics in Microgravity Protein Crystal Growth

The dynamic coupling between mass transport and incorporation of growth units into the surface of a crystal growing from solution in microgravity is used to derive quantitative information on the crystal growth kinetics. To this end, new procedures for experiment preparation, interferometric data processing and model fitting have been developed. The use of experimental data from the bulk diffusive maw transport together with a model for steady state stagnant crystal growth allows the detailed quantitative understanding of the kinetics of both the concentration depletion zone around the crystal and the growth of the crystal interface. The protein crystal used in the experiment is shown to be growing in the mixed kinetic regime (0.2 x 10(exp -6) centimeters per second less than beta R/D less than 0.9 x 10(exp -6) centimeters per second).

Otalora, F.↗

NASA NTRS2003

Coulomb Stress Accumulation along the San Andreas Fault System

Stress accumulation rates along the primary segments of the San Andreas Fault system are computed using a three-dimensional (3-D) elastic half-space model with realistic fault geometry. The model is developed in the Fourier domain by solving for the response of an elastic half-space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth. This approach is then applied to the San Andreas Fault system using published slip rates along 18 major fault strands of the fault zone. GPS-derived horizontal velocity measurements spanning the entire 1700 x 200 km region are then used to solve for apparent locking depth along each primary fault segment. This simple model fits remarkably well (2.43 mm/yr RMS misfit), although some discrepancies occur in the Eastern California Shear Zone. The model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates. In addition, shear and normal stresses along the major fault strands are used to compute Coulomb stress accumulation rate. As a result, we find earthquake recurrence intervals along the San Andreas Fault system to be inversely proportional to Coulomb stress accumulation rate, in agreement with typical coseismic stress drops of 1 - 10 MPa. This 3-D deformation model can ultimately be extended to include both time-dependent forcing and viscoelastic response.

Smith, Bridget↗

NASA NTRS2005

Regression Analysis of Long-term Profile Ozone Data Set from BUV Instruments

Frith, Stacey↗

NASA NTRS1981

Constraints on the origin and interior structure of the major planets

Fitting models to the external gravity field of the major planets (Uranus, Neptune, Jupiter, and Saturn), it is found that certain interior characteristics may be common to all four. For Uranus and Neptune, a model with a central iron-silicate core of approximately four earth masses, an 'ice' layer of H2O, CH4, and NH3 in solar proportions of about 10 earth masses, and an H2-He atmosphere of about 1-2 earth masses gives a good fit to available constraints, including heat flow measurements. Models of Jupiter and Saturn have cores very similar to those of Uranus and Neptune; the H2-He layer, however, is markedly more extensive. Modes of origin that are consistent with these features are discussed. Models of this type predict a considerable enrichment of deuterium relative to primordial solar abundances in Uranus and Neptune. Such enrichment is not observed in Uranus; implications are discussed for interior structure and origin.

Hubbard, W. B.↗

NASA NTRS1974

A line-blanketed model stellar atmosphere of Sirius

The method of artificial absorption edges is applied to a grid of model atmospheres to include the effects of hydrogen and metal line blanketing on the atmospheric structure. A best-fit model of Sirius is obtained which is free of the discrepancies inherent in unblanketed or partially blanketed models. All major spectral features calculated from the model agree with the corresponding observed features to within the observational uncertainty. Some additional properties of the models are discussed.

Fowler, J. W.↗

NASA NTRS1994

IUE spectra and optical imaging of the oxygen-rich supernova remnant N132D

We present new optical Charge Coupled Devices (CCD) interference filter imagery and International Ultraviolet Explorer (IUE) spectroscopy for the oxygen-rich supernova remnant N132D in the Large Magellanic Cloud. The optical images show a wealth of structure, and comparison with an archival Einstein High Resolution Imager (HRI) X-ray image shows that a few optical features have X-ray counter-parts, but in general there is little correlation between X-ray and optical features. The IUE spectra at two positions show strong lines of carbon and oxygen, with lines of neon, magnesium, silicon, and helium also present and variable in relative intensities. We use optical data for N132D from Dopita & Tuohy (1984) with our UV observations to compare with shock models (both with and without thermal conduction) and X-ray photoionization model calculations. While none of the model fits is entirely satisfactory, the generally weak UV emission relative to optical disagrees with the general character of shock model predictions and indicates that photoionization is the dominant excitation mechanism for the UV/optical emission. This conclusion is similar to what was found for E0102 - 7219, the oxygen-rich remnant in the Small Magellanic Cloud. We derive rough abundances for the emitting material in N132D, compare to stellar nucleosynthesis models, and discuss the implications for its precursor. A precursor near 20 solar mass is consistent with the data.

Blair, William P.↗