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NASA NTRS1982

Detection of nonthermal continuum radiation in Saturn's magnetosphere

A detailed analysis of high resolution wideband data from the Voyager 1 and 2 plasma wave receivers has revealed the presence of heretofore undiscovered nonthermal continuum radiation trapped within the Saturnian magnetosphere. The discovery of Saturnian trapped continuum radiation fills a disturbing void in the Saturnian radio spectrum. On the basis of observations at both the earth and Jupiter it was expected that continuum radiation should be a pervasive signature of planetary magnetospheres in general. Special processing of the Voyager 1 plasma wave data at Saturn has now confirmed the existence of weak emissions that have a spectrum characteristic of trapped continuum radiation. Similar radiation was also detected by Voyager 2; however, in this case it is not certain that Saturn was the only source. Considerable evidence exists which suggests that Saturn may have been immersed in the Jovian tail during Voyager 2 encounter, so that Jupiter may provide an additional source of the continuum radiation detected by Voyager 2.

Kurth, W. S.↗

NASA NTRS1982

Generation of nonthermal continuum radiation in the magnetosphere

Generation of electromagnetic continuum radiation from electrostatic fluctuations near the upper hybrid resonance frequency has been calculated by using cold plasma theory in an inhomogeneous plasma near the plasmapause. It is shown that both the polarization and the amplitude of electromagnetic radiation are in good quantitative agreement with spacecraft observations for nonthermal continuum radiation.

Okuda, H.↗

NASA NTRS1983

Thermal conduction and heating by nonthermal electrons in the X-ray halo of M87

A hydrostatic model for the X-ray halo around the giant elliptical galaxy M87 is presented. It is shown that by taking into account the processes of thermal conduction, and nonthermal heating by relativistic electrons in the radio lobes, a self-consistent hydrostatic model can be constructed. There is no need to invoke radiative accretion or the suppression of thermal conductivity.

Tucker, W. H.↗

NASA NTRS1983

Nonthermal escape of hydrogen and deuterium from Venus and implications for loss of water

As the dominant nonthermal mechanism for the escape of hydrogen in past Venus atmospheres, the charge exchange of H(+) with H would have provided an escape flux close to the diffusion-limiting value for H-mixing ratios up to 0.002 at the homopause, which also marks the onset of hydrodynamic flow. Charge exchange therefore represents a viable mechanism through which Venus could have lost up to an earth-equivalent ocean of water from its atmosphere over geologic time. Present Venus atmosphere estimates are based on in situ Pioneer Venus mission measurements, and assumptions in the course of extrapolation to past atmospheres have been with respect to the nature of the bulge, circulation pattern, and ion temperature.

Kumar, S.↗

NASA NTRS1983

O(1S) from dissociative recombination of O2(+) - Nonthermal line profile measurements from Dynamics Explorer

The measurements reported were made of the O(1S) emission line profile at 5577 A at high spectral resolution with the Fabry-Perot interferometer on the Dynamics Explorer spacecraft. It is found that the line profile has marked nonthermal characteristics in the nightglow. A simple collisional relaxation model is used to analyze the nighttime emission line profiles, measured in the equatorial region. The branching ratio is inferred for the dissociative recombination of O2(+) leading to O(1S). The result reveals that the O(1S) + O(1D) channel is favored over the O(1S). The result reveals that the O(1S) + O(1D) channel is favored over the O(1S) + O(3P) channel by a factor of 4; this does not agree with the ratio reported by Hernandez (1971). It is noted, however, that the result is consistent with the active channel for O(1S) production being via the 1Sigma u + repulsive state of O2, as suggested by the theoretical calculations of Guberman (1983). In addition, a value is obtained for the excitation exchange cross section for O(1S).

Killeen, T. L.↗

NASA NTRS1984

Gas dynamics in the impulsive phase of solar flares. I Thick-target heating by nonthermal electrons

A numerical investigation is carried out of the gas dynamical response of the solar atmosphere to a flare energy input in the form of precipitating nonthermal electrons. Rather than discussing the origin of these electrons, the spectral and temporal characteristics of the injected flux are inferred through a thick-target model of hard X-ray bremsstrahlung production. It is assumed that the electrons spiral about preexisting magnetic field lines, making it possible for a one-dimensional spatial treatment to be performed. It is also assumed that all electron energy losses are due to Coulomb collisions with ambient particles; that is, return-current ohmic effects and collective plasma processes are neglected. The results are contrasted with earlier work on conductive heating of the flare atmosphere. A local temperature peak is seen at a height of approximately 1500 km above the photosphere. This derives from a spatial maximum in the energy deposition rate from an electron beam. It is noted that such a feature is not present in conductively heated models. The associated localized region of high pressure drives material both upward and downward.

Nagai, F.↗

NASA NTRS1984

Fast variations in high-energy X-rays from solar flares and their constraints on nonthermal models

The results of a quantitatve analysis of quantitative the temporal properties of over 7000 hard X-ray solar flares are presented. The data were obtained with the Hard X-ray Burst Spectrometer of the SMM satellite. The data reveal examples of hundreds of fast spikes with durations of less than 1 s. The spectra of a well-observed fast spike were compared with the predicted spectra of a detailed nonthermal electron beam model, in order to determine upper limits for electron beam parameters and path lengths. The theoretical spectra were found to be consistent with the observed spectra when the separation of between the accretion site and the thick target was greater than 10 to the 8th cm. It is shown that if the separation is much greater than 3 x 10 to the 9th cm, the electrons with high pitch angles (greater than 60 deg) are either not emitted or are mirrored before they reach the footpoints.

Kiplinger, A. L.↗

NASA NTRS1985

Electron acceleration in solar flares and the transition from nonthermal to thermal hard X-ray phases

Observations are reviewed which indicate that hard X-rays during the impulsive phase of a flare typically start with a primarily nonthermal phase which undergoes a transition to a primarily thermal phase as the flare progresses. Recent theoretical work on the modified two-stream instability as an efficient electron accelerator and modeling of thermal hard X-ray sources is considered. A scenario which is termed the dissipative thermal model is proposed to explain the observations. Fast tearing modes occurring in a loop give rise to cross-field ion motion. This in turn excites the modified two-stream instability which converts about 50 percent of the ion energy into accelerated electrons along the loop as long as the plasma beta is less than 0.3. These electrons impact the chromosphere and boil off a part of it which rises up the loop. This density increase coupled with the temperature increase due to tearing causes the beta to increase beyond 0.3 and efficient electron acceleration ceases. This leads to the primarily thermal phase.

Smith, D. F.↗

NASA NTRS1985

Nonthermal electron-positron pair production and the 'universal' X-ray spectrum of active galactic nuclei

A nonthermal model for power-law X-ray and gamma-ray sources is considered. An initial, primary distribution of relativistic electrons is injected and cooled via Compton scattering of soft photons (produced either externally or by the synchrotron mechanism). The scattered photons, constituting a primary gamma-ray source, produce electron-positron pairs that act as a secondary electron injection, which in turn produce a secondary photon spectrum. Pairs formed by a part of the photon spectrum optically thin to pair production are taken into account. The distribution of particles and photons is obtained, and numerical results as well as analytical solutions to certain special cases are presented. For the case of a delta-function primary electron injection it is found that the photon spectrum in the X-ray region is well approximated by a power law, with the energy spectral index alpha(X) lying in the relatively narrow range 0.5-0.9 as the compactness parameter L(x)/R (where L(x) is the X-ray luminosity and R is the source radius) varies over many orders of magnitude. This is proposed as a possible mechanism to explain the universal X-ray spectra observed from active galactic nuclei.

Zdziarski, A. A.↗

NASA NTRS1985

A new property of the small-scale nonthermal motions in quiescent prominences

Small-scale nonthermal motions are a characteristic feature of astrophysical atmospheres, which have been studied on the basis of an interpretation of high-dispersion spectral line observations. The present paper is concerned with a study based on the close-lying lines Mg double-lambda 5173, 5167 and Fe(+) lambda 5169, in quiescent prominences. A summary of Mg b2, b4, and Fe(+) b3 prominence measurements and derived model profile parameters is presented in a table, while another table contains a summary of derived temperature and microvelocity parameter values. The most important result of the reported study is the finding that the Fe(+) b3 profiles in quiescent prominences are systematically narrower than one would expect from the Mg b2 profile widths and the assumption that both lines arise in regions of common T and v.

Landman, D. A.↗

NASA NTRS1985

Thermal infrared and nonthermal radio - Remarkable correlation in disks of galaxies

A tight, linear correlation is established between the far-infrared flux measured by IRAS and the nonthermal radio flux density (at 1.4 GHz) from disks of spiral galaxies. This correlation defines a ratio of infrared to radio fluxes that is characteristic of star formation activity. Galaxies with nuclear starbursts seem to follow the correlation. If the far-infrared is reradiated luminosity from young massive stars, then the supernova remnants alone account for less than 10 percent of the radio emission. These results indicate a close coupling between dust heating and cosmic-ray generation and confinement in a wide range of conditions.

Helou, G.↗

NASA NTRS1985

Possible nightside source dominance in nonthermal radio emissions from Uranus

Desch and Kaiser (1984) have formulated a radiometric Bode's law from which they have attempted to estimate the low frequency, nonthermal radio power of the magnetosphere of Uranus. It is shown here that, if Uranus possesses a magnetosphere, it is more likely that the radio emission is from the nightside as opposed to the dayside as assumed by Desch and Kaiser. A nightside source for the radio emissions would radically alter the predicted time for direct observations of the emissions.

Curtis, S. A.↗

NASA NTRS1986

On magnetic field stochasticity and nonthermal line broadening in solar flares

Observations of nonthermal line broadening seen in solar flares by the Solar Maximum Mission satellite are discussed in light of recent results on the generation of magnetic field stochasticity. It is shown that a consistent model for the data can be constructed by assuming that the observations signal the destruction of an ambient magnetohydrodynamic equilibrium.

Antonucci, E.↗

NASA NTRS1986

Gamma-ray burst spectra from photon-deficient Compton scattering by nonthermal electrons

Consideration is given to a model of gamma-ray burst sources based on repeated Compton scatterings of soft photons by relativistic nonthermal electrons. Relativistic electrons which are continuously produced in the source radiate the total power L supplied to them. Higher order Compton scatterings, which occur when L(soft) is much less than L make the model distinct. The spectrum having an X-ray energy index of about one results from the superposition of the spectral components from several orders of Compton scattering; the change of the slope at several hundred keV arises from the Klein-Nishina decline of the scattering cross section.

Zdziarski, Andrzej A.↗

NASA NTRS1987

Collisional heating by nonthermal electrons in a tapered magnetic loop

The behavior of nonthermal electrons ejected into a tapered magnetic loop, under the action of both Coulomb collisional and magnetic field gradient forces is studied. An approximate analytic formula for the heating rate as a function of distance along the loop is developed, and found to be in good agreement with exact numerical solutions of the relevant equations. Such a formula is useful as a source term in many situations, such as hydrodynamic simulations of atmospheric response to flare energy input.

Chandrashekar, S.↗

NASA NTRS1987

Spectroscopy and nonthermal processes

Stellar spectra are analyzed to determine nonthermal processes for cool stars. A shock wave crossing model is supported by a study of the behavior of absorption and emission spectra. The shock waves are attributed to atmospheric kinetics. Circumstellar spectral lines are studied for information about gaseous circumstellar layers. The description of stellar envelopes is carried on through circumstellar dust. Characteristic properties of polarization in the dust are described in the case of specific stars, emphasizing narrowband observations in Mira, semiregular, and supergiant stars. Finally, the direct approach to measuring the angular diameters of stars and mapping the distribution of circumstellar dust and gas by lunar occultation or interferometry is discussed, using two prototype stars, an M supergiant and a dusty carbon star.

Querci, Monique↗

NASA NTRS1987

The discovery of nonthermal radio emission from magnetic Bp-Ap stars

In a VLA survey of chemically peculiar B- and A-type stars with strong magnetic fields, five of the 34 stars observed have been identified as 6 cm continuum sources. Three of the detections are helium-strong early Bp stars (Sigma Ori E, HR 1890, and Delta Ori C), and two are helium weak, silicon-strong stars with spectral types near A0p (IQ Aur = HD 34452, Babcock's star = HD 215441). The 6 cm luminosities L6 (ergs/s Hz) range from log L6 = 16.2 to 17.9, somewhat less than the OB supergiants and W-R stars. Three-frequency observations indicate that the helium-strong Bp stars are variable nonthermal sources.

Drake, Stephen A.↗

NASA NTRS1988

Rapid temporal evolution of radiation from nonthermal electrons in solar flares

Solutions of the time dependent Fokker-Planck equation was found for accelerated electrons undergoing Coulomb collisions in a magnetized, fully ionized plasma. An exact solution was found for arbitrary pitch angle and energy distribution in a uniform background plasma. Then, for an inhomogeneous plasma, a solution was found for particles with small pitch angles. These solutions were used to calculate the temporal evolution of bremsstrahlung X-rays from short bursts of nonthermal electron beams, and these spectra were compared with observed high time resolution spectra of short timescale solar hard X-ray bursts. It is shown that the observed softening in time of the spectra rules out a homogeneous background and therefore the possibility of electrons being confined to the corona either because of converging magnetic field or high densities. The inhomogeneous solution was also applied to a model with constant coronal density and exponentially rising chromospheric density. The spectra are shown to be consistent with that produced by a collimated beam of electrons accelerated in the corona with certain given conditions. These conditions could be violated if large pitch angle electrons are present.

Lu, Edward T.↗