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At least 1,135 records · Page 63
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NASA NTRS1988

The driver in flares and coronal mass ejections: Magnetic expansion

Chromospheric filaments, and hence the sheared magnetic fields that they trace, are observed to erupt in flares and coronal mass ejections. In the eruption, the filament-traced field is seen to expand in volume. For frozen-in magnetic field and isotropic expansion, the magnetic energy in a flux tube decreases as the flux tube expands. The amount of expansion of the magnetic field and the corresponding decrease in magnetic energy in a filament-eruption flare and/or coronal mass ejection can be estimated to order of magnitude from the observed expansion of the erupting filament. This evaluation for filament-eruption events in which the filament expansion is clearly displayed gives decreases in magnetic energy of the order of the total energy of the accompanying flare and/or coronal mass ejection. This simple expanding flux tube model can also fit the observed acceleration of coronal mass ejections, if it is assumed that the increase in mechanical energy of the mass ejection comes from the magnetic energy decrease in the expansion. These results encourage the view that magnetic expansion such as seen in filament eruptions drives both the plasma particle energization in flares and the bulk mass motion in coronal mass ejections.

Moore, Ronald L.↗
NASA NTRS1989

Infrared photometry of comet Wilson (1986l) at two epochs

1-20 micron filter photometry of comet Wilson (1986l) was obtained on May 29 - June 2, 1987 (r = 1.36 AU) and January 11 -14, 1988 (r = 3.75 AU) at the NASA Infrared Telescope Facility. The thermal emission at 1.36 AU has been fit with a model for a size distribution of small absorbing grains; the size and the optical properties of the dust (color, albedo, and thermal emission spectrum) appear typical of short period comets at similar r. No 10 micron silicate feature was evident. The dust-production rate on June 1 was about 5 x 10 to the 5th g/s. The geometric albedo of comet Wilson at r = 3.75 AU appears to be higher than that of comets measured at smaller r, after phase effects have been accounted for. Such albedos seem typical of comets at larger heliocentric distances.

Hanner, Martha S.↗
NASA NTRS1989

Ginga observations of dipping low mass X ray binaries

Ginga observations of several low mass X ray binaries displaying pronounced dips of variable depth and duration in their X ray light curves are analyzed. The periodic occultation of the central X ray source by azimuthal accretion disk structure is considered. A series of spectra selected by intensity from the dip data from XB1916-053, are presented. The effects of a rapidly changing column density upon the spectral fitting results are modeled. EXO0748-676 was observed in March 1989 for three days. The source was found to be in a bright state with a 1 to 20 keV flux of 8.8 x 10 (exp -10) erg/sqcms. The data include two eclipses, observed with high time resolution.

Smale, Alan P.↗
NASA NTRS1990

Jovian H2 dayglow emission (1978-1989)

The IUE data set accumulated through 10 years of Jovian equatorial observations is used to measure the long-term temporal variation of the H2 dayglow emission. The model that best fits the data indicates a possible correlation between long-term solar activity and the Jovian H2 emission in the region 1500-1700 A between 1978 and 1989, which spans the decline in solar activity for solar cycle 21 and the rise in solar activity accompanying solar cycle 22. The magnitude of the observed variation is closer to that of the solar Ly-alpha flux than the 10.7 cm radio flux. Short-wavelength H2 band emission intensity is inconsistent with the amount of long-wavelength emission but may be reconciled if relatively low-energy excitation or fluorescence of solar radiation is invoked. No persistent longitudinal feature analogous to the H I Ly-alpha can be identified.

Mcgrath, M. A.↗
NASA NTRS1991

Neural-Network Simulator

F77NNS (FORTRAN 77 Neural Network Simulator) computer program simulates popular back-error-propagation neural network. Designed to take advantage of vectorization when used on computers having this capability, also used on any computer equipped with ANSI-77 FORTRAN Compiler. Problems involving matching of patterns or mathematical modeling of systems fit class of problems F77NNS designed to solve. Program has restart capability so neural network solved in stages suitable to user's resources and desires. Enables user to customize patterns of connections between layers of network. Size of neural network F77NNS applied to limited only by amount of random-access memory available to user.

Mitchell, Paul H.↗
NASA NTRS1989

Interstellar extinction at 10-20 microns

The IRAS low-resolution spectra (LRS) spectra of 117 stars of excellent signal/noise with optically thin silicate dust shells were analyzed. The stellar continua (assumed to be a cool black body) were subtracted, and the resulting dust shell spectra were fit with simple models F(sub lambda) assuming uniform mass loss and dust temperature as a function of distance from the star, calculated using the optical constants for silcates of Draine (1985). From the comparison of the spectra and the models, functions for the emissivity, kappa(sub lambda), were derived.

Simpson, Janet P.↗
NASA NTRS1991

GPS measurements of deformation associated with the 1987 Superstition Hills earthquake: Evidence for conjugate faulting

Large station displacements observed from Imperial Valley Global Positioning System (GPS) campaigns are attributed to the November 24, 1987 Superstition Hills earthquake sequence. Thirty sites from a 42 station GPS network established in 1986 were reoccupied during 1988 and/or 1990. Displacements at three sites within 3 kilometers of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (M(sub S) 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 + or - 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 + or - 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 x 10(exp 25) dyne-cm and 2.3 x 10(exp 25) dyne-cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest the post seismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using Singular Value Decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of non-seismic displacement is observed across the Imperial Valley, which is attributed in part to interseismic plate-boundary deformation.

Larsen, Shawn↗
NASA NTRS1991

The ultraviolet spectrum of Scorpius X-1 as observed by IUE - 1978-1988

The IUE observations of Sco X-1 taken prior to September 1988 are summarized. These data are tested for variability associated with orbital motion and for correlations between the various observables. The continuum spectra are fitted to simple models for accretion disk atmospheres which demonstrate that X-ray heating dominates the outer regions of the accretion disk. Various scenarios for producing the observed line strengths and ratios are discussed.

Kallman, T. R.↗
NASA NTRS1991

Ganymede and Callisto - Surface textural dichotomies and photometric analysis

Complete solar phase curves of the Ganymede and Callisto leading and trailing hemispheres, which have been obtained by reducing Voyager imaging observations and combining them with ground-based telescopic data, are presently fit to scattering models in order to derive hemispherical values of the single scattering albedo, the single particle phase function (SPPF), the compaction state (CS) of the optically active portion of the regolith, and the mean slope angle of macroscopic features. While Callisto's leading side is composed of particles that are more strongly backscattering than the trailing side, no hemispheric differences are found in the CS, surface roughness, or SPPF.

Buratti, Bonnie J.↗
NASA NTRS1992

Global positioning system measurements of deformations associated with the 1987 Superstition Hills earthquake - Evidence for conjugate faulting

Large station displacements observed from Imperial Valley Global Positioning System (GPS) compaigns are attributed to the November 24, 1987 Superstition Hills earthquake sequence. Thirty sites from a 42 station GPS network established in 1986 were reoccupied during 1988 and/or 1990. Displacements at three sites within 3 kilometers of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (M(sub S) 6.6) within a preexisting GPS network. Best-fitting uniform slip models of rectangular dislocations in an elastic half-space indicate 130 + or - 8 cm right-lateral displacement along the northwest-trending Superstition Hills fault and 30 + or - 10 cm left-lateral displacement along the conjugate northeast-trending Elmore Ranch fault. The geodetic moments are 9.4 x 10 (exp 25) dyne-cm and 2.3 x 10 (exp 25) dyne-cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest the post seismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using Singular Value Decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of non-seismic displacement is observed across the Imperial Valley, which is attributed in part to interseismic plate-boundary deformation.

Larsen, Shawn↗
NASA NTRS1991

Radial propagation of solar energetic particles assuming large scattering mean free paths in the interplanetary medium

Observations of two flares which fit the diffusive model by Beeck et al. (1987) are employed to determine whether large solar-particle events can be explained by the mechanisms of small impulsive events. It is shown that the injection of particles at the source is extended in time and lasts more than approximately 10 hours. The extended injection at the sun is hypothesized to be the reason that large solar particles do not demonstrate the 'pulse/wake' behavior associated with the small impulsive events.

Mason, G. M.↗
NASA NTRS1992

The kinematics of the molecular gas in Centaurus A

The CO (2-1) emission along the inner dust lane of Centaurus A, observed with the Caltech Submillimeter Observatory on Mauna Kea, shows the molecular gas to be in a thin disk, with a velocity dispersion of only about 10 km/s. The observed line profiles are broadened considerably due to beam smearing of the gas velocity field. The profile shapes are inconsistent with planar circular and noncircular motion. However, a warped disk in a prolate potential provides a good fit to the profile shapes. The morphology and kinematics of the molecular gas is similar to that of the ionized material, seen in H-alpha. The best-fitting warped disk model not only matches the optical appearance of the dust lane but also agrees with the large-scale map of the CO emission and is consistent with H I measurements at larger radii.

Quillen, A. C.↗
NASA NTRS1992

Development of a four-dimensional variational analysis system using the adjoint method at GLA. I - Dynamics

Recent developments in the field of data assimilation have pointed to variational analysis (essentially least-squares fitting of a model solution to observed data) using the adjoint method as a new direction that holds the potential of major improvements over the current optimal interpolation method. This paper describes the initial effort in the development of a 4D variational analysis system. Although the development is based on the Goddard Laboratory for Atmospheres General Circulation Model (GCM), the methods and procedures described in this paper can be applied to any model. The adjoint code that computes the gradients needed in the analysis can be written directly from the GCM code. An easy error-detection technique was devised in the construction of the adjoint model. Also, a method of determining the weights and the preconditioning scales for the cases where model-generated data, which are error free, are used as observation is proposed. Two test experiments show that the dynamics part of the system has been successfully completed.

Chao, Winston C.↗
NASA NTRS1992

Polarization effects and multipolarization SAR

Imaging radar polarimeters are usually implemented using a Synthetic Aperture Radar (SAR) approach to give a high resolution image in two dimensions: range and azimuth. For each pixel in the image a polarimetric SAR gives sufficient information to characterize the polarimetric scattering properties of the imaged area (or target) as seen by the radar. Using a polarimetric SAR system as opposed to a single-polarization SAR system provides significantly more information about the target scattering mechanisms and allows better discrimination between different types of surfaces. In these notes a brief overview of SAR polarimetry is offered. The notes are intended as a text to accompany a lecture on SAR polarimetry as part of an AGARD-NATO course. Covered in the notes are the following: the polarization properties of electromagnetic waves; the concepts of radar scattering and measuring radar backscatter with a SAR; polarization synthesis; scattering matrix, Stokes matrix, and covariance matrix representations of polarimetric SAR data; polarization signature plots; design and calibration of polarimetric SAR systems; polarization filtering for target detection; fitting a simple model to polarimetric SAR measurements of naturally occurring features; and supervised classification of polarimetric SAR data.

Freeman, Anthony↗
NASA NTRS1993

IUEAGN: A database of ultraviolet spectra of active galactic nuclei

In 13 years of operation, IUE has gathered approximately 5000 spectra of almost 600 Active Galactic Nuclei (AGN). In order to undertake AGN studies which require large amounts of data, we are consistently reducing this entire archive and creating a homogeneous, easy-to-use database. First, the spectra are extracted using the Optimal extraction algorithm. Continuum fluxes are then measured across predefined bands, and line fluxes are measured with a multi-component fit. These results, along with source information such as redshifts and positions, are placed in the IUEAGN relational database. Analysis algorithms, statistical tests, and plotting packages run within the structure, and this flexible database can accommodate future data when they are released. This archival approach has already been used to survey line and continuum variability in six bright Seyfert 1s and rapid continuum variability in 14 blazars. Among the results that could only be obtained using a large archival study is evidence that blazars show a positive correlation between degree of variability and apparent luminosity, while Seyfert 1s show an anti-correlation. This suggests that beaming dominates the ultraviolet properties for blazars, while thermal emission from an accretion disk dominates for Seyfert 1s. Our future plans include a survey of line ratios in Seyfert 1s, to be fitted with photoionization models to test the models and determine the range of temperatures, densities and ionization parameters. We will also include data from IRAS, Einstein, EXOSAT, and ground-based telescopes to measure multi-wavelength correlations and broadband spectral energy distributions.

Pike, G.↗
NASA NTRS1993

An astrophysics data program investigation of spatial structure of supernova remnants

The final report on astrophysics data program investigation of spatial structure of supernova remnants for the period 1 Aug. 1989 to 31 Jul. 1991 is presented. The goal of the project was the study of the spatial structure of supernova remnants (SNR's) as observed in the x-ray band. A number of software tools were developed for the analysis: (1) a program to fit various geometric models to high resolution x-ray data, and (2) programs for Fourier Transform analysis of clumping in SNR's. These programs were applied to high resolution imager (HRI) data on the young galactic SNR's Tycho and Kepler with some success.

Hughes, John P.↗
NASA NTRS1992

Tests of shock chemistry in IC 443G

Eight molecular species, in the hot dense clump IC 443G, believed to be impacted by the shock wave from the SNR IC 443, are investigated. The clump consists of two distinct regions, one relatively cool, and one hotter and denser. Region 1 contains CO, HCO(+), HCN, and CN, whose abundances may be explained either by ion-molecule chemistry, or by a D shock of 60-90 km/s, passing through a clump of about 100,000/cu cm. Region 2 gives rise to SiO, CS, SO, and H2CO, and requires an ND shock of 5-15 km/s passing through a region of about 1,000,000/cu cm. Observed fractional abundances fit ND shock models if L is about 6.6 x 10 exp 15 cm. In general, observed line widths vary inversely with derived excitation density, while centroid velocities of all species are essentially identical.

Turner, B. E.↗