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NASA NTRS1979

Energetic electron bursts in the magnetopause electron layer and in interplanetary space

The magnetopause electron layer in the distant magnetotail is an annular region encircling the magnetopause in which bursts of tailward-streaming energetic (E 200 keV) electrons are almost continuously present. Sunward-streaming electron bursts with time scales and energy spectral indices similar to those of layer bursts are sometimes observed in interplanetary space upstream of the Earth. Evidence is presented to show that the layer bursts and the interplanetary bursts have a common source. With the aid of a new coordinate system, geocentric interplanetary medium coordinates, appropriate for describing the access of energetic charged particles in the inner magnetosheath to a spacecraft located in interplanetary space, it is shown that the interplanetary bursts occur predominantly on the sunward extension of the field lines associated with the magnetopause electron layer.

Bieber, J. W.↗

NASA NTRS1980

The position and polarization of Type V solar bursts

Observations of the position and polarization of Type V solar radio bursts and their preceding Type III bursts are presented. The polarization, frequency range, source position, source movement, source size and brightness temperature of the bursts were measured using a 24-220 MHz spectropolarimeter, an 8-8000 MHz spectrograph and a three-frequency radioheliograph. Type V radiation is frequently found to have the opposite sense of circular polarization from that of the preceding Type III burst, with a degree of polarization similar to that of harmonic Type III radiation. A reversal of polarization is not observed when the accompanying Type III burst has no fundamental-harmonic structure, or when the Type V radiation is poorly developed. Possible mechanisms for the reversal are examined, including opposite magnetic field directions in Type III and V bursts, changes in mode coupling and a change in the mode of emission from o-mode for Type III to x-mode for Type V, and conditions needed for the mode change which is considered the most likely mechanism, are determined.

Dulk, G. A.↗

NASA NTRS1980

Repeatable, multiple-peaked structure in Type I X-ray bursts

A distinct, identifiable, multiple-peaked structure has been observed in Type I X-ray bursts from three sources. At energies below 6 keV, the light curves look like typical Type I bursts. At higher energies, the burst is double-peaked, with both the depth of the dip and the separation between the peaks increasing with energy. A light curve of the energy-integrated intensity shows no distinct double peak, suggesting that only a single energy release occurs. Blackbody fits to the evolving burst spectra yield changing radii and temperatures, inversely correlated, during the early part of the burst. The physical interpretation of these changes is uncertain. Burst decay spectra yield relatively constant radii with decreasing temperatures. It is proposed that Compton scattering may be responsible for the dips in the higher-energy light curves by shifting photons to lower energies.

Hoffman, J. A.↗

NASA NTRS1984

Voyager observations of Jovian millisecond radio bursts

Voyager Planetary Radio Astronomy data collected over 30-day intervals centered on the two close encounters with Jupiter were utilized to study the characteristics of millisecond-duration radio bursts (s-bursts) at frequencies between 5 and 15 MHz. In this frequency range, s-bursts are found to occur almost independently of Central Meridian Longitude and to depend entirely on the phase of Io with respect to the observer's planetocentric line of sight. Individual bursts typically cover a total frequency range of about 1.5 to 3 MHz, and they are usually strongly circularly polarized. Most bursts in a particular s-burst storm will exhibit the same polarization sense (either right-hand or left-hand), and there is some evidence for a systematic pattern in which one polarizations sense is preferred over the other as a function of Io phase and Central Meridian Longitude. These data are all suggestive of a radio source that is located along the instantaneous Io flux tube and that extends over a linear dimension of 5000 km along the field lines in both the northern and southern Hemispheres.

Alexander, J. K.↗

NASA NTRS1984

Chorus-related electrostatic bursts at Jupiter and Saturn

Analyses of the wideband plasma wave data obtained by Voyagers 1 and 2 at Jupiter and Saturn have revealed electrostatic bursts similar to those recently discovered at earth in association with whistler-mode chorus. In all three magnetospheres the bursts are characterized by sporadic emissions near or slightly below the electron plasma frequency with bandwidths ranging from 10 percent to more than 50 percent of the center frequency. The events found at Jupiter occur in the middle magnetosphere during both the dayside as well as the early morning passes. At Saturn, the bursts occurred in the outer regions of the magnetosphere during the dayside pass. In each of the events analyzed, evidence exists for modulation of the electrostatic bursts by a low frequency wave, presumably chorus. One of the observations gained at Jupiter includes the detection of a low-frequency band at the proper frequency for chorus. Detailed waveform analysis confirms that this band does, indeed, modulate the electrostatic bursts. Based on the present understanding of the terrestrial observations it is believed that the electrostatic bursts are generated by an electron beam trapped in Landau resonance with the chorus. Previously announced in STAR as N83-37042

Reinleitner, L. A.↗

NASA NTRS1984

Voyager observations of Jovian millisecond radio bursts

Alexander, J. K.↗

NASA NTRS1984

Physics of gamma-ray bursts

Attention is given to the accumulating evidence for the view that gamma-ray bursts come from strongly magnetic neutron stars, discussing the physical properties of the emission region and the radiation processes expected in strong magnetic fields, and emphasizing that the observed burst spectra require that the emission region be optically thin. This entails that the energy of the emitting plasma and/or the plasma itself be continuously replenished during a burst, and that the cooling time scale of the emitting plasma be much shorter than the observed duration of the bursts. This characteristic of the cooling time scale implies that the burst intensity and spectrum can vary on extremely short time scales, and that the burst duration must have a separate explanation. It is emphasized that synchrotron emission is favored as the gamma-ray production mechanism; it is the only mechanism capable of satisfying the optical thinness constraint while producing the observed luminosity.

Lamb, D. Q.↗

NASA NTRS1983

Observation of an absorption feature in a gamma ray burst spectrum

A gamma ray burst was detected on March 25, 1978 by the High Energy X-ray and Low Energy Gamma Ray Experiment on HEAO-1. The burst spectrum shows an absorption feature at 55 + or - 5 keV with an equivalent width of 13 + or - 3 keV, values commensurate with those of similar features observed by the KONUS experiment. The burst spectrum also is characterized by a hard component extending from about 0.25-6 MeV. This component can be interpreted in terms of a fireball model for gamma ray bursts, which places the distance to the source at 1 kpc. The integrated fluence of the burst between 0.025 and 6 MeV is 1.5 x 10 to the -5th ergs/sq cm. The burst source has been localized to within a degree of RA = 237.5 deg and Dec = 76.2 deg.

Hueter, G. J.↗

NASA NTRS1985

A statistical study of solar type III bursts and auroral kilometric radiation onsets

Simultaneous occurrences of type III solar radio bursts and auroral kilometric radiation were observed by Calvert (1981) using ISEE 1 spectrograms. Calvert presented evidence suggesting that the incoming type III burst stimulates the onset of auroral kilometric radiation (AKR). This paper presents a statistical study of the correlation between type III bursts and auroral kilometric radiation. A superposed epoch analysis was performed on as many as 186 type III events. The type III bursts were detected by the ISEE 3 spacecraft on the sunward side of the earth. At the same time the IMP 8 spacecraft was used to detect onsets of kilometric radiation on the nightside of the earth. For each event the intensities measured by ISEE 3 (type III intensities) were subtracted from the intensities measured by IMP 8 (type III and possible AKR intensities). The resulting intensities for each event were then added to determine if kilometric radiation was preferentially observed following a type III burst. This analysis was performed at frequencies of 100, 178, and 500 kHz. The results of this study show that a statistically significant correlation exists between incoming type III bursts from the sun and kilometric radiation from the earth.

Farrell, W. M.↗

NASA NTRS1987

18 MHz interferometry of Non-Io-C L-bursts

In order to investigate previous discrepancies in source size measurements, 18-MHz interferometric data of L-bursts from a Non-Io-C storm have been collected during the summer of 1986. Variations noted in the fringe visibility of the Jovian L-bursts indicate that the source size changes in time and that the emission region can sometimes be resolved with a relatively modest baseline. The results suggest that all Jovian source sizes are smaller than 10 arcsec (and are possibly smaller than 0.1 arcmin) and that it is more probable that L-burst emission during a Jovian storm will evolve into S-bursts than that S-bursts will become L-bursts.

Phillips, J. A.↗

NASA NTRS1989

Model interpretation of type III radio burst characteristics. II - Temporal aspects

A model of the radio emission region for kilometric type II bursts is used to interpret systematic variations observed in the temporal behavior of the burst parameters. The temporal behavior of the burst parameters observed by the ISEE-3 spacecraft is reviewed, and it is pointed out that the phase and modulation of the antenna signal vary in a systematic way with both time and observing frequency. The source azimuth is observed to drift with time, with the magnitude and sense of the drift depending on the location of the radio source relative to the observer. The modulation factor usually decreases uniformly with time and is frequently peaked near the burst onset. The model of the radio emission region is developed and used to obtain the intensity, phase, and modulation of the radio signal. Model results are used to show how the behavior of the burst parameters are related to attributes of the source region. It is shown that the temporal behavior of the radio parameters for an observed type II burst is well represented by the model.

Reiner, M. J.↗

NASA NTRS1990

Hard X-rays and associated weak decimetric bursts

Results are presented of observations (made with the Hard X-Ray Burst Spectrometer on the SMM and with the Itapetinga antenna) that unambiguously show associations of hard X-ray bursts with RS-type III bursts between 16:46 UT and 16:52 UT on July 9, 1985. The comparison between the hard X-ray and the radio observations shows a clear association, in at least 13 cases, of the hard X-ray and the decimetric RS bursts. On the average, the X-ray peaks were delayed from the peak of the RS bursts at 1.6 Ghz by about 400 msec. It is shown that, if the time comparisons are made between the peak of the radio pulse and the start of the X-ray burst, the delays are consistent with an electron travel time with a velocity of about 0.3 c from the 800 MHz plasma level to the lower corona, assuming that the radio emission is at the second harmonic.

Sawant, H. S.↗

NASA NTRS1991

Gamma-ray bursts from colliding strange stars

The rate of collisions between the neutron stars is about 0.0001/yr in the galaxy and about 0.00001/yr within the Hubble distance. The collisions are the final phases of binary orbit decay driven by gravitational radiation and may produce gamma-ray bursts detectable at extragalactic distances. If strange stars exist then their collisions must release about 10 to the 50th ergs in gamma rays over 0.2 s. Such events should be detectable out to 1 Gpc with the current instruments. The distance to the majority of gamma-ray bursts is not known at this time. The Burst and Transient Source Experiment (BATSE) on Gamma Ray Observatory should determine the distance scale by determining the angular distribution of very weak bursts. If the majority of gamma-ray bursts turn out to be extragalactic, and if their distances are about 1 Gpc, then the collisions between strange stars may be the least speculative events that might account for so energetic bursts.

Haensel, P.↗

NASA NTRS1991

Pressure vessel burst test program. II

The current status is disucssed of a program to study the characteristics of blast waves and fragmentation generated by ruptured gas-filled pressure vessels. Current methods for assessing vessel safety and burst parameters are briefly reviewed, and pneumatic burst testing operations and testing results are examined. A comparison is made with current methods for burst assessment. It is tentatively concluded that, at close distances, vessel burst overpressures are less than those of high-explosive (HE) blasts with equivalent energy and are greater than HE far from the vessel. The impulse appears to be the same for both vessel bursts and equivalent energy HE blasts. The functional relationship between shock velocity and overpressure ratio appears to be the same for vessel bursts as for HE blasts. The initial shock overpressure appears to be much less than vessel pressure and may be found using the one-dimensional shock tube equation.

Cain, Maurice R.↗

NASA NTRS1992

High-energy spectral breaks in gamma-ray bursts

Model fits are presented for 18 gamma-ray burst spectra from 100 keV to 27 MeV made with the BATSE spectroscopy detectors on the Compton Gamma Ray Observatory. Most of the bursts are well fitted as power laws with spectral indices between -1.36 and -2.29; however, five bursts show definite departures from a simple power-law fit at high energies. Three of these bursts are well fitted with broken power-law spectra and break energies of from 400 to 690 keV, such as might arise from photon-photon interactions. If so, then the source compactness and hence distance will be sharply constrained. Two of the bursts have spectra with sharply confined slope changes and are well fitted with broken power-law spectra with break energies of 1.2 and 1.6 MeV at peak, such as might arise from photon-magnetic field interactions. If so, then these spectral breaks provide strong evidence for the existence of high magnetic fields in the burst emission region.

Schaefer, Bradley E.↗

NASA NTRS1992

Statistics of gamma-ray bursts - Homogeneous spherical models

Early reports from the BATSE experiment seem to suggest that the observed gamma-ray bursts occur in a highly flattened region of space. This inference, however, is contradicted by the apparent isotropy of the bursts. Here, it is demonstrated that this apparent dilemma can be resolved in a straightforward manner if bursts occur in a bounded spherical volume centered about the observer. Such a geometrical distribution of the bursts can be consistent with the apparent isotropy of the bursts. Two different models in which the burst population is uniformly distributed in a bounded spherical region of space are considered.

Wasserman, Ira↗

NASA NTRS1992

Control of VLF burst activity in the nightside ionosphere of Venus by the magnetic field orientation

A burst identificaton method developed by Ho et al. (1991) is used to analyze data on the nightside ionosphere of Venus in order to determine whether VLF bursts observed there are better interpreted as whistler mode waves or ion acoustic waves. The correlation between burst occurrence and the angle between the magnetic field and the radial direction, as well as the spacecraft flight direction are examined. Narrow-band 100-Hz bursts are found to be more frequently associated with radial magnetic fields while wideband signals are more frequently associated with horizontal fields. Under the assumption of vertical propagation, the normalized 100-Hz burst rate inside the resonance cone is larger than that outside. The burst rate inside the resonance cone dominates the altitude distribution. By assuming vertical propagation, the 100-Hz signals clearly divide into two populations. One is whistler mode propagating inside the resonance cone. The other is a nonpropagating mode outside the resonance cone which decreases quickly with altitude with a scale height of about 20 km.

Ho, C.-M.↗

NASA NTRS1992

Gamma-ray bursts; Proceedings of the Workshop, Univ. of Alabama, Huntsville, Oct. 16-18, 1991

Recent developments in gamma-ray burst research are discussed with emphasis on new observations of gamma-ray bursts made by the Compton Gamma-Ray Observatory, Granat, Ginga, Pioneer Venus Orbiter, Prognoz, and Phobos. The papers are grouped under the following headings: general, historical, recent spacecraft observations; spatial and intensity distributions; galactic and extragalactic models; spectral observations; time profiles; and search for counterparts. Specific topics discussed include an upper limit on the luminosity of cosmological gamma-ray bursts; low-mass X-ray binaries and gamma-ray bursts; spectral evolution of gamma-ray bursts; ionization of gamma-ray-burst environments; and BATSE observations of bremsstrahlung from electron precipitation events.

Paciesas, William S.↗