ELECTRON ACCELERATION IN SOLAR-FLARES BY FAST-MODE WAVES - COULOMB COLLISIONS

We consider solar-flare electron acceleration from thermal to hard X-r ay-producing energies by MHD fast-mode waves. Fast-mode waves are an e fficient acceleration agent when isotropizing pitch-angle scattering i s present. Pitch-angle scattering increases the acceleration efficienc y, since fast-mode waves affect only the parallel (with respect to the ambient magnetic field) electron energy and produce a parallel energi zation rate that is proportional to the square of the magnetic moment. We remove the isotropy condition and consider specifically the pitch- angle scattering that results from electron-electron Coulomb collision s in an impulsive solar flare environment. We compare results for the case of (1) no Coulomb collisions, (2) energy redistribution by Coulom b collisions in the parallel direction only, and (3) Coulomb pitch-ang le scattering with energy loss. We find that Coulomb pitch-angle scatt ering strongly enhances the efficiency of fast-mode wave acceleration and produces bulk energization of the ambient electrons.