Particle acceleration in impulsive solar flares
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 10^32 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 Alfv\'en 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 Alfv\'en
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.