S. R. Kane

Acceleration and confinement of energetic particles in the 1980 june 7 solar flare

Abstract : Pulsations with large amplitude and duration have been observed during the hard X-ray and microwave radio bursts associated with the 7 June 1980 (approx. 0312 UT) solar flare. The high time resolution measurements of 20-800 keV X-rays were made with the X-ray spectrometers aboard the ISEE-3 and P78-1 spacecraft. The radio measurements, covering metric to microwave wavelengths, were made at the Nobeyama and Toyokawa observatories in Japan. The temporal evolution of the X-ray and radio spectra and the polarization and spatial structure of the microwave source have been examined. The following interpretation is found to be consistent with the observations: (1) the variations in the electron acceleration/injection spectrum are responsible for the observed variations in the hard X-ray and microwave emissions; (2) the locations of the hard X-ray and microwave sources are probably different, the X-ray source being located at a lower altitude. (Author)

A new type of repetitive behavior in a high-energy transient

The high-energy burster GB 790107 has been observed to repeat, on the order of 100 times, between August 13, 1978 and June 27, 1986, with most repetitions occurring in the latter part of 1983. Here, these activities are described, discussing intensity and time-of-occurrence information covering a seven-year span of International Cometary Explorer (ICE) data. Arguments for the reality of this activity and its association with the GB 790107 source are presented. A moderately precise location derived from ICE, SMM, Pioneer Venus Orbiter, and Venera 13 Signe data is given and related to the published GB 790107 location. 9 references.

Spatial structure of greater than 100 keV X-ray sources in solar flares

Steroscopic X-ray observations of the 1979 November 5 solar flares permitted the first measurements of the altitude structure of the impulsive and gradual sources of > or approx. = 100 keV X-rays. It is found that the brightness of the impulsive as well as the gradual hard X-ray sources decreases rapidly with increase in altitude above the photosphere. Most of the X-ray emission seems to originate at altitudes < or approx. =2500 km, indicating the models of hard X-ray sources with low (< or approx. =10/sup 10/ cm/sup -3/) ion density are not consistent with the observations.

Observation of an impulsive solar X-ray burst from a coronal source

New observations of the spatial, spectral, and temporal structure of an impulsive hard X-ray source in a behine-the-limb solar flare have been made with high time resolution, hard X-ray detectors aboard two spacecraft, the International Sun Earth Explorer 3 (ISEE 3) and the Pioneer Venus Orbiter (PVO), which were separated in heliographic longitude by approx.12/sup 0/.5. The principal findings are that (1) the coronal part of the X-ray source is approx.600 times less intense than the lower-altitude part of the source; and (2) the coronal X-ray observations are consistent with a power-law electron spectrum which extends down to approx.5 keV.

Stereoscopic observations of a solar flare hard X-ray source in the high corona

The vertical structure of the impulsive and gradual hard X-ray sources in high coronae and the characteristics of the impulsive soft X-ray emission are investigated on the basis of PVE, ICE, and GOES observations of the energetic flare on February 16, 1984. The average photon spectra observed by these instruments during the impulsive and gradual hard X-ray bursts are summarized. A comparison of these unocculted and partially occulted spectra shows that the sources of the impulsive hard X-ray (greater than about 25 keV) and impulsive soft X-ray (2-5 keV) emissions in this flare extended to coronal altitudes greater than about 200,000 km above the photosphere. At about 100 keV, the ratio of the coronal source brightness to the total source brightness was 0.001 during the impulsive phase and less than about 0.01 during the gradual hard X-ray burst. The sources of the gradual hard X-ray burst and gradual soft X-ray burst were almost completely occulted, indicating that these sources were located at heights less than 200,000 km above the photosphere. 47 refs.

Directivity of 100 keV-1 meV photon sources in solar flares

Stereoscopic observations of 0.1-1.0 MeV photon sources in solar flares made with spectrometers aboard the ISEE 3 (Third International Sun Earth Explorer) and PVO(Pioneer Venus Orbiter) were analyzed to determine the directivity of the photon sources and its possible dependence on photon energy. During the period 1978 October 1-1980 October 31, a total of 44 solar flares were observed simultaneously by the two instruments. Of these, 39 flares were in full view of both the instruments, the remaining five being partially occulted by the photosphere from the line of sight of at least one instrument. The view angles theta and theta of the PVO and ISEE 3 instruments with respect to the outward solar radius at the flare site varied from one flare to another and were in the range 9-88. The difference theta - theta 1 between the two view angles varied from 1 to 66. The observations of differential photon energy spectra averaged over > or = 16 s do not indicate any systematic directivity. In most flares the directivity j (theta = 90)/j(theta = 0) of 0.1-1.0 MeV photon sources is found to be < or = 2.5.

Stereoscopic Observations of Solar Hard X-Ray Flares Made by Ulysses and Yohkoh

The Solar X-Ray/Cosmic Gamma-Ray Burst Experiment aboard the interplanetary spacecraft Ulysses has provided extensive observations of solar hard X-ray flares from a variety of angles with respect to the Sun-Earth line. During the period 1991 October-1993 June, Ulysses observed 13 flares that were also observed by the X-ray instruments aboard the Japanese satellite Yohkoh located near Earth. At least 12 flares were in full view of both the spacecraft. Eight flares, for which hard X-ray spectra were available, are examined to determine the directivity of the 20-125 keV hard X-ray sources in solar flares. They include one flare for which the view angles of Ulysses and Yohkoh were 80° and 25°, respectively. No evidence of systematic directivity was found within the uncertainty (a factor of ~2) of these measurements. These and other observations of directivity at higher energies are consistent with a nearly isotropic distribution of energetic electrons in most solar flares.

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.

On the interpretation of gamma-ray burst continua and possible cyclotron absorption lines

Most gamma‐ray bursts have spectra which are fit by an optically thin thermal bremsstrahlung shape. We show that the small emitting volume implied by the observation of spectral features identified as cyclotron absorption lines from a 1012 Gauss field prevent thermal bremsstrahlung from providing sufficient luminosities to account for the observed fluxes unless the optical depth to Compton scattering is large. In addition, we demonstrate that complicated detector response functions can introduce artifacts which mimic absorption features below 60 keV unless the absolute gain of the detector is established and correctly applied in analysis. By comparing the continuum as observed by the Berkeley/Los Alamos ISEE‐3 gamma‐ray detector with the continuum observed by the Konus experiments we find a lack of agreement, suggesting that the gain of one of these experiments may be incorrect. Thus, a great deal of caution should be exercised in interpreting the apparent low energy features as cyclotron lines.

Hard X-ray and high-frequency decimetric radio observations of the 4 April 2002 solar flare

Abstract Hard X-ray and high frequency decimetric type III radio bursts have been observed in association with the soft X-raysolar flare (GOES class M 6.1) on 4 April 2002 (∼1532 UT). The flare apparently occurred ∼ 6 degrees behind the east limb of the Sun in the active region NOAA 9898. Hard X-ray spectra and images were obtained by the X-ray imager on RHESSI during the impulsive phase of the flare. The Brazilian Solar Spectroscope and Ondrejov Radio Telescopes recorded type III bursts in 800–1400 MHz range in association with the flare. The images of the 3–6, 6–12, 12–25, and 25–50 keV X-ray sources, obtained simultaneously by RHESSI during the early impulsive phase of the flare, show that all the four X-ray sources were essentially at the same location well above the limb of the Sun. During the early impulsive phase, the X-ray spectrum over 8–30 keV range was consistent with a power law with a negative exponent of ∼ 6. The radio spectra show drifting radio structures with emission in a relatively narrow (Δf ≤ 200 MHz) frequency range indicating injection of energetic electrons into a plasmoid which is slowly drifting upwards in the corona.