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

The hard X-ray/gamma-ray (HXR/GR) impulsive burst on 1991 June 30 (~0255 UT) was associated with a flare which occured between 2 and 12 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 (Dingus et al. 1994; Vilmer et al. 1999). However, this event is one of the giant flares reported by Kane et al. (1995). It was observed by the Solar Xray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses, located 135 east of the Earth-Sun line. GRB measured the total (greater than)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 (greater than)28 keV emission. (gamma) decreased from ~5.4 to ~2.6 and R varied from ~20% at the beginning of the event down to (less than01% 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.