GAMMA-RAYS FROM PION DECAY - EVIDENCE FOR LONG-TERM TRAPPING OF PARTICLES IN SOLAR-FLARES

We analyze the energy spectrum and time dependence of the 50 MeV to 2 GeV gamma rays observe from the 1991 June 11 solar flare. We show that the emission detected at the late phase of this flare with EGRET on the Compton Gamma-Ray Observatory can be explained by a model in which the bulk of the particles were accelerated during the impulsive phase and subsequently trapped in coronal magnetic loops. We fit the observed spectrum with a combination of pion decay radiation and primary electron bremsstrahlung. We compare the 1991 June 11 data with data for the 1982 June 3 and 1991 June 15 flares from which pion decay emission was also observed. The fact that the fluxes from these three flares are ordered in time in accordance with the predicted time dependence of emission produced by trapped particles provides support for the model.