Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT

Details Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT

Jun 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...403.1157t&link_type=abstract

Astronomy and Astrophysics, v.403, p.1157-1163 (2003)

Astronomy and Astrophysics

Astrophysics

1

Sun: Activity, Sun: Flares, Sun: Particle Emission, Sun: X-Rays, Gamma-Rays

Scientific paper

The hard X-ray/gamma-ray (HXR/GR) impulsive burst on 1991 June 30 ( ~ 0255 UT) was associated with a flare which occured between 2degr and 12degr behind the east limb of the Sun. The partially occulted HXR/GR emission from this flare was detected at up to 100 MeV by three instruments on Earth-orbiting spacecraft: the Burst and Transient Source Experiment (BATSE) and the Energetic Gamma-Ray Experiment (EGRET) on CGRO and by the Payload for High Energy Burst Spectroscopy (PHEBUS) on GRANAT. As seen from the two spacecraft in Earth orbit, the size of the burst corresponds to that of a moderate electron-dominated GR event \citep{Din:al-94, Vil:al-99}. However, this event is one of the giant flares reported by \citet{Kan:al-95}. It was observed by the Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses, located 135degr east of the Earth-Sun line. GRB measured the total >28 keV HXR emission from the flare. In this paper we combine HXR observations by GRB and BATSE in order to determine the time evolution of the power-law index gamma of the photon spectrum of the partially occulted HXR emission seen by BATSE and of the fraction R of the partially occulted to the total >28 keV emission. gamma decreased from ~ 5.4 to ~ 2.6 and R varied from ~ 20% at the beginning of the event down to <1% at its maximum. These results indicate that the spatial distribution of the HXR sources was complex, and evolved in the course of the event. While the HXR emission detected by GRB was almost entirely produced at the footpoints of loops by thick-target interactions, a fraction of the HXR emission seen by BATSE likely originated in the unocculted, low density, portion of the HXR emitting loops. The data also show that a small fraction ( ~ 10%) of the HXR emission detected by BATSE in Earth orbit was radiated by a thick-target source on the visible disk.

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