GAMMA-RAY EMISSION AND ELECTRON ACCELERATION IN SOLAR-FLARES

Recent observations have extended the spectra of the impulsive phase o f flares to the GeV range. Such high-energy photons can be produced ei ther by electron bremsstrahlung or by decay of pions produced by accel erated protons. In this paper we investigate the effects of processes which become important at high energies. We examine the effects of syn chrotron losses during the transport of electrons as they travel from the acceleration region in the corona to the gamma-ray emission sites deep in the chromosphere and photosphere, and the effects of scatterin g and absorption of gamma rays on their way from the photosphere to sp ace instruments. These results are compared with the spectra from so-c alled electron-dominated flares, observed by GRS on the Solar Maximum Mission, which show negligible or no detectable contribution from acce lerated protons. The spectra of these flares show a distinct steepenin g at energies below 100 keV and a rapid falloff at energies above 50 M eV. Following our earlier results based on lower energy gamma-ray flar e emission we have modeled these spectra. We show that neither the rad iative transfer effects, which are expected to become important at hig her energies, nor the transport effects (Coulomb collisions, synchrotr on losses, or magnetic field convergence) can explain such sharp spect ral deviations from a simple power law. These spectral deviations from a power law are therefore attributed to the acceleration process. In a stochastic acceleration model the low-energy steepening can be attri buted to Coulomb collision and the rapid high-energy steepening can re sult from synchrotron losses during the acceleration process.