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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
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NASA NTRS1987

Polarization effects on hard target calibration of lidar systems

The theory of hard target calibration of lidar backscatter data, including laboratory measurements of the pertinent target reflectance parameters, is extended to include the effects of polarization of the transmitted and received laser radiation. The bidirectional reflectance-distribution function model of reflectance is expanded to a 4 x 4 matrix allowing Mueller matrix and Stokes vector calculus to be employed. Target reflectance parameters for calibration of lidar backscatter data are derived for various lidar system polarization configurations from integrating sphere and monostatic reflectometer measurements. It is found that correct modeling of polarization effects is mandatory for accurate calibration of hard target reflectance parameters and, therefore, for accurate calibration of lidar backscatter data.

Kavaya, Michael J.↗
NASA NTRS1986

Solar gradual hard X-ray bursts - Observations and an interpretation

A recent study of solar gradual hard X-ray bursts is summarized. The data are interpreted in terms of a model involving the acceleration and trapping of electrons in post flare loop systems following coronal mass ejections. A controversy about the classification of the metric continuum that typically accompanies gradual hard X-ray events is addressed.

Cliver, E. W.↗
NASA NTRS1987

On the hard X-ray spatial structure during the impulsive phase of solar flares

A simplified form of the bremsstrahlung cross-section is used to obtain an analytic expression for the intensity of electron-beam-produced hard X-ray emission with depth in solar flares which can be used in a first-order analysis of imaging data. The results indicate that the conditions for the appearance of bright footpoint emission, in terms of loop parameters such as density and length, are much less restrictive than previously suggested. The analysis shows that the observed footpoint structure of many flares in hard X-rays is consistent with the thick-target bombardment model, and that the intensity of the footpoint emission relative to the spatially integrated flux can be used as a diagnostic tool of coronal column density.

Emslie, A. Gordon↗
NASA NTRS1988

The hard X-ray burst spectrometer event listing 1980-1987

This event listing is a comprehensive reference for the Hard X-ray bursts detected with the Hard X-ray Burst Spectrometer on the Solar Maximum Mission from the time of launch 14 February 1980 to December 1987. Over 8600 X-ray events were detected in the energy range from 30 to approx. 600 keV with the vast majority being solar flares. The listing includes the start time, peak time, duration and peak rate of each event.

Dennis, B. R.↗
NASA NTRS1987

Hard X-ray emission in electron-heated solar flares - A comparison of nonthermal and thermal contributions

The spatial structure of hard X-ray emission during the impulsive phase of electron-heated solar flares is calculated. Both direct nonthermal bremsstrahlung and the thermal bremsstrahlung arising from the heated plasma are considered. The results indicate that the spread of nonthermal emission into the upper parts of the loop, through evaporation of the chromospheric target, may be more important than the appearance of a hot thermal source in the corona. The effects of varying the viewing angle to the flare loop, and of finite-size resolution element, are also considered, and results are compared with observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer.

Brown, David G.↗
NASA NTRS1987

Hard X-ray and radio emission at the onset of great solar flares

A study of the onset phase of ten great hard X-ray bursts is presented. It is shown from hard X-ray and radio observations in different wavelength ranges that the energization of the electrons proceeds on a global time-scale for some tens of seconds. In nine of the bursts, two phases of emission can be distinguished during the onset phase: the preflash phase (during which emission up to an energy limit ranging from some tens of keV to 200 keV is observed) followed ten to some tens of seconds later by the flash phase (where the count rate in all detector channels rises simultaneously to within some seconds). For two of the events, strong gamma-ray line emission is observed and is shown to start close to the onset of the flash phase.

Klein, K.-L.↗
NASA NTRS1988

The effects of mixing of the ejecta on the hard X-ray emissions from SN 1987A

The X-ray and gamma-ray emissions expected from SN 1987A have been calculated, taking into account mixing of material in the ejecta. Nuclear gamma rays emitted by Co-56 are scattered down to the hard X-ray band by multiple Compton scatterings. Nomoto's 11E1Y6 model for the ejecta of SN 1987A is used. X-ray light curves in the 10-30 keV band and spectra above 20 keV calculated with an inner mixed region of 5 + or - 1 solar mass are consistent with the observations performed with the Ginga satellite and the Kvant/Roentgen mission. On the basis of this comparison, further evolutions of the hard X-ray, gamma-ray, and optical/infrared emissions are discussed.

Ebisuzaki, Toshikazu↗
NASA NTRS1987

A long-duration balloon payload for hard X-ray and gamma-ray observations of the sun

A balloon payload designed to study the processes of energy release, particle acceleration, and heating of the active corona in hard X-ray microflares and normal flares is described. An array of liquid nitrogen-cooled germanium detectors together with large area phoswich scintillation detectors provide the highest sensitivity (about 500 sq cm) and energy resolution (not greater than 0.7 keV) ever achieved for solar hard X-ray (about 15-600 keV) measurements. These detectors were flown in February 1987 from Australia on a long duration radiation controlled balloon flight (LDBF) which provided 12 days of observations before cutdown in Brazil. The payload includes solar cells for power, pointing, and navigation sensors, a microprocessor controlled data system with VCR tape storage, and transmitters for GOES and ARGOS spacecraft. This successful flight illustrates the potential of LDBFs for solar flare studies.

Lin, R. P.↗
NASA NTRS1988

Temporal correlations between impulsive ultraviolet and hard X-ray bursts in solar flares observed with high time resolution

The impulsive phase of solar flares has been simultaneously observed in the ultraviolet O V line, the UV continuum, and hard X-rays with a time resolution of 0.128 s by the SMM satellite. A close time correspondence between the three impulsive components is found, with the best correlation being at the peak of the impulsive phase. Individual bursts or fast features in the O V and the UV continuum are shown to lag behind the corresponding hard X-ray features. None of the considered energy transport mechanisms (thermal conduction, a nonthermal electron beam, electron hole boring, UV radiation, and Alfven waves) are able to consistently account for the observed temporal correlations.

Cheng, Chung-Chieh↗
NASA NTRS1988

Hard X-ray observations of ultraluminous infrared galaxies

X-ray flux upper limits of 2-10 keV for ultraluminous infrared galaxies were drawn from the HEAO A-1 data base. The hard X-ray luminosities of these sources are much weaker relative to their total luminosities than would be expected for Seyfert 1 galaxies or quasi-stellar objects. Because of the low level of interstellar extinction for hard X-rays, this result suggests that the ultraluminous galaxies are not powered by embedded QSOs that are otherwise similar to other QSOs. Three other possibilities are: (1) the infrared galaxies may contain a form of X-ray-quiet active nucleus; (2) the X-ray sources in active nuclei may not turn on until after the circumnuclear gas has cleared; or (3) the bulk of the infrared luminosity in these galaxies may be generated by intense circumnuclear star formation.

Rieke, G. H.↗
NASA NTRS1988

Experimental and simulation studies of hard contact in force reflecting teleoperation

Experiments and simulations of a single-axis force-reflecting teleoperation system have been conducted to investigate the problem of contacting a hard environment and maintaining a controlled force in teleoperation in which position is fed forward from the hand controller (master) to the manipulator (slave), and force is fed back to the human operator through motors in the master. The simulations, using an electrical circuit model, reproduce the behavior of the real system, including effects of human operator biomechanics. It is shown that human operator properties, which vary as a result of different types of grasp of the handle, affect the stability of the system in the hard-contact task. The effect of a heavier grasp on the handle is equivalent to increased hand-controller velocity damping in terms of the systems stability in the contact task, but control system damping sufficient to guarantee stable contact results in perceptible sluggishness of the control handle's response in free motion. These results suggest that human operator biomechanics must be taken into account to guarantee stable and ergonomic performance of advanced teleoperators.

Hannaford, Blake↗
NASA NTRS1989

Impulsiveness and energetics in solar flares with and without type II radio bursts - A comparison of hard X-ray characteristics for over 2500 solar flares

The hard X-ray characteristics of more than 2500 solar flares are used to study the relative size, impulsiveness, and energetics of flares with and without type II radio bursts. A quantitative definition of the hard X-ray impulsiveness is introduced, which may be applied to a large number of events unambiguously. It is found that the flares with type II bursts are generally not significantly larger, more impulsive, or more energetic than those without type II bursts. Also, no evidence is found to suggest a simple classification of the flares as either 'impulsive' or 'gradual'. Because type II bursts are present even in small flares with relatively unimpulsive energy releases, it is concluded that changes in the ambient conditions of the solar atmosphere causing an unusually low Alfven speed may be important in the generation of the shock wave that produces type II radio bursts.

Pearson, Douglas H.↗
NASA NTRS1989

Computational studies of hard-body and 3-D effects in plume flows

Axisymmetric and three-dimensional, multi-nozzle plume flows around generic rocket geometries are investigated with a three-dimensional Navier-Stokes solver to study the interactive effects between hard body and the plume. Time-asymptotic, laminar, ideal-gas solutions obtained with a two-factor, flux-split scheme and a diagonal, upwind scheme are presented. Computed solutions to three-dimensional, multi-nozzle problems and single-nozzle, axisymmetric problems demonstrate flow field features including three-dimensionality and hard-body effects. Geometry and three-dimensional effects are shown to be significant in multi-nozzle flows.

Venkatapathy, Ethiraj↗
NASA NTRS1988

Hard X-ray and low-energy gamma-ray spectrometers

Basic principles of operation and characteristics of scintillation and semi-conductor detectors used for solar hard X-ray and gamma-ray spectrometers are presented. Scintillation materials such as NaI offer high stopping power for incident gamma rays, modest energy resolution, and relatively simple operation. They are, to date, the most often used detector in solar gamma-ray spectroscopy. The scintillator BGO has higher stopping power than NaI, but poorer energy resolution. The primary advantage of semi-conductor materials such as Ge is their high-energy resolution. Monte-Carlo simulations of the response of NaI and Ge detectors to model solar flare inputs show the benefit of high resoluton for studying spectral lines. No semi-conductor material besides Ge is currently available with adequate combined size and purity to make general-use hard X-ray and gamma-ray detectors for solar studies.

Gehrels, N.↗
NASA NTRS1989

Pre-pulses: Signature of a trigger process in short (less than 60 secs) solar hard x ray flares

The continuing study of short hard x ray events (less than 60 sec duration) from the Solar Maximum Mission (SMM) Hard X ray Burst Spectrometer (HXRBS) instrument has revealed a unique feature. A well-separated distinctly identifiable, narrow (2 to 6 sec wide) pulse occurs prior to the start of the longer-flare lasting emission activity. Light curves are presented for eight events showing this feature. The pre-pulses show symmetrical rise and fall times. Spectral evolution of the pre-pulses are presented and their evolution compared to that of the main event spectra. It is argued that this feature be the elementary flare burst (de Jager, 1978). These pre-pulses could be a signature of the magnetic reconnection phenomenon discussed by Sturrock et al., (1984).

Deasi, U.↗
NASA NTRS1989

Hard emission at late times from SN 1987A

A model for the explosion that has been successful in predicting and explaining the evolution of SN 1987A during its first one and one-half years is used to calculate the future photometric evolution of the supernova in the UV-optical-IR, X-ray, and gamma-ray bands using Monte Carlo techniques. Special attention is given to the contribution from radioactive isotopes other than Co-56, notably Co-57, Ti-44, and Na-22, and to the possible appearance, probably within the next year, of X-rays from an accreting magnetic neutron star at the center of the supernova. The signature of a radio pulsar like the Crab is also considered. In both cases the time history of the bolometric light curve may be a more sensitive diagnostic than the existence of either pulses or hard emission, but will become confused in the near future by contributions from the rare radioactivities. A source as faint as the pulsed emission of the Crab pulsar will never be discernable in the bolometric light curve. Pulsed hard emission is best sought near 30 keV near the end of 1989 (and thereafter), although more sensitive instrumentation than hitherto employed may be necessary.

Woosley, S. E.↗
NASA NTRS1990

Balloon-borne hard X-ray observations of SN 1987A

The region of the LMC containing SN 1987A was observed during a balloon flight of a hard-X-ray telescope on April 8-10, 1988. Significant continuum emission was detected in the 45-400-keV range and attributed to SN 1987A. Compared to an observation with the same instrument in October 1987, the source intensity decreased by about 50 percent, while the spectral shape remained qualitatively the same. This may represent the first clear indication that the hard-X-ray emission is entering the declining phase expected as the ejecta become optically thin to the Co-56 gamma rays.

Fishman, G. J.↗