NEW PROMISE FOR ELECTRON BULK ENERGIZATION IN SOLAR-FLARES - PREFERENTIAL FERMI ACCELERATION OF ELECTRONS OVER PROTONS IN RECONNECTION-DRIVEN MAGNETOHYDRODYNAMIC TURBULENCE

The hard X-ray luminosity of impulsive solar flares indicates that ele ctrons in the low corona are bulk energized to energies of order 25 ke V. LaRosa & Moore pointed out that the required bulk energization coul d be produced by cascading MHD turbulence generated by Alfvenic outflo ws from sites of strongly driven reconnection. LaRosa, Moore, gr Shore proposed that the compressive component of the cascading turbulence d issipates into the electrons via Fermi acceleration. However, for this to be a viable electron bulk energization mechanism, the rate of prot on energization by the same turbulence cannot exceed the electron ener gization rate. In this paper we estimate the relative efficiency of el ectron and proton Fermi acceleration in the compressive MHD turbulence expected in the reconnection outflows in impulsive solar flares. We f ind that the protons pose no threat to the electron energization. Part icles extract energy from the MHD turbulence by mirroring on magnetic compressions moving along the magnetic held at the Alfven speed. The m irroring rate, and hence the energization rate, is a sensitive functio n of the particle velocity distribution. In particular, there is a low er speed limit V-min approximate to V-A, below which the pitch-angle d istribution of the particles is so highly collapsed to the magnetic fi eld in the frame of the magnetic compressions that there is no mirrori ng and hence no Fermi acceleration. For coronal conditions, the proton thermal speed is much less than the Alfven speed and proton Fermi acc eleration is negligible. In contrast, nearly all of the electrons are super-Alfvenic, so their pitch-angle distribution is nearly isotropic in the frame of the magnetic compressions. Consequently, the electrons are so vigorously mirrored that they are Fermi accelerated to hard X- ray energies in a few tenths of a second by the magnetic compressions on scales of 10(5)-10(3) cm in the cascading MHD turbulence. We conclu de that dissipation of reconnection-generated MHD turbulence by electr on Fermi acceleration plausibly accounts for the electron bulk energiz ation in solar flares.