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Session 26 - Solar Flares II.
Oral session, Monday, June 10
We determined elemental abundances in the solar atmosphere using SMM gamma ray line data. The abundances show a pronounced FIP (first ionization potential) effect. The abundance ratios of the low FIP to high FIP elements (Mg/O, Si/O, Fe/O) are enhanced compared to their photospheric values and are consistent with coronal values. On the other hand, the value of Ne/O (both high FIP elements) is somewhat higher than the coronal value obtained from solar energetic particle data and the generally adopted photospheric value. To avoid an unreasonably high value of Ne/O, a power law energy spectrum of accelerated particles extending down to about 1 MeV/nucl is needed. This implies an approximate equipartition between the total energies carried by accelerated electrons and ions, contrary to the previous belief that the bulk of the flare energy resides in electrons. We also found that the composition of the interacting particles is similar to that of the accelerated particles from impulsive flares, i.e. is enriched in ^3He and heavy elements. Strong enhancement of heavy elements is also required to account for the gamma ray data obtained with the PHEBUS instrument on GRANAT during the extremely intense, behind the limb flare on 1991 June 1. We conclude that, independent of the flare type (impulsive or gradual), the gamma rays are produced by particles that are accelerated by the same mechanism that operates in impulsive flares, most likely through gyroresonant wave particle interactions.
Program listing for Monday