The Giant 1991 June 1 Flare: Evidence for Gamma‐Ray Production in the Corona and Accelerated Heavy Ion Abundance Enhancements from Gamma‐Ray Spectroscopy

We investigated the implications of the gamma-ray line emission observed with Granat/PHEBUS from the behind-the-limb flare on 1991 June 1. We showed that thin target interactions are required to account for the very high observed ratio of the nuclear line emission in the 1.1-1.8 MeV and 4.1-7.6 MeV energy bands, which are populated predominantly by gamma rays from de-excitations of Ne-Fe and C-O, respectively. We found that the composition of the accelerated particles that produce gamma rays in this extremely powerful gradual flare clearly shows the heavy-element abundance enhancements characteristic of acceleration via resonant wave-particle interactions. Furthermore, the heavy-element abundance enhancements increase with lime, reaching, toward the end of the flare, the highest values observed in space from impulsive solar energetic particle events. We showed that the energy deposited in the coronal interaction region is comparable to the largest total accelerated particle energy contest in previously observed gamma-ray flares. However, this deposited energy is small in comparison with the energy content in greater than 20 keV electrons inferred from hard X-ray observations with Ulysses for which the 1991 June 1 Bare was in full view. Approximate energy equipartition between accelerated electrons and ions then implies that the total ion energy content is much larger than the portion deposited in the corona, requiring a short ion residence time tau and a low ambient density n(H) in the coronal interaction region (n(H) tau similar or equal to 2.5 x 10(9) cm(-3) s).