Electron Acceleration in Solar Flares by Fast Mode Waves: Quasi‐linear Theory and Pitch‐Angle Scattering

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Abstract
It has been shown recently that small-amplitude, long-wavelength, fast mode waves can accelerate a large number of electrons from the ambient thermal distribution to relativistic energies on subsecond timescales and can thereby account for the general properties of electron acceleration in impulsive solar flare energy release fragments. This prior study employed quasi-linear theory as well as the assumption that ancillary pitch-angle scattering kept the electron distribution isotropic. In light of the high efficiency of the resulting stochastic acceleration process, we question the necessity of pitch-angle scattering in the model and the validity of quasi-linear theory in describing the wave-particle interaction. We find that quasi-linear theory does predict accurately the behavior of electrons, even when the turbulence has an energy density about equal to the ambient magnetic field energy density, and that pitch-angle scattering is a necessary condition for acceleration. We also discuss the more general issue of modeling continuous wave spectra by discrete ones for use in test particle simulations.