J. A. Miller University of Alabama in Huntsville, Huntsville, Alabama, USA
Impulsive solar flares are among the most energetic events in the solar system, releasing up to 1032 ergs of energy over timescales as short as a few minutes. A great deal of this energy appears in the form of relativistic electrons and ions, which exceed by several times the number of particles originally in the flare volume. The issue of how these particles are accelerated rapidly from thermal to relativistic energies is a basic problem in flare research, and we propose that stochastic acceleration by cascading low-amplitude MHD waves is an attractive and simple solution. Specifically, weak MHD waves (composed of the shear Alfvén and fast modes), whose wavelength can be as large as the scalesize of the flare, are generated by either reconnection or magnetic field restructuring during the primary energy release phase. Upon cascading to smaller wavelengths each mode resonates preferentially with increased efficiency with either electrons or ions, until eventually the stochastic acceleration rate exceeds the Coulomb drag and particles can be accelerated out of the thermal tail and to relativistic energies. Electrons interact with the compressive magnetic field of the fast mode waves via Landau resonance, which yields a process known as transit-time acceleration. Ions interact with the transverse electric field of the shear Alfvén waves via cyclotron resonance, which also leads to rapid stochastic acceleration. We will quantitatively discuss this theory in detail using results from a self-consistent quasilinear simulation, and demonstrate how the essential features of flare particle acceleration can be accounted for, including the heavy ion abundance enhancements.