A NEW PATH FOR THE ELECTRON BULK ENERGIZATION IN SOLAR-FLARES - FERMIACCELERATION BY MAGNETOHYDRODYNAMIC TURBULENCE IN RECONNECTION OUTFLOWS

We recently proposed that a magnetohydrodynamic (MHD) turbulent cascad e produces the bulk energization of electrons to similar to 25 keV in the impulsive phase of solar flares (LaRosa and Moore 1993). In that s cenario, (1) the cascading MHD turbulence is fed by shear-unstable Alf venic outflows from sites of strongly driven reconnection in the low c orona, and (2) the electrons are energized by absorbing the energy tha t flows down through the cascade. We did not specify the physical mech anism by which the cascading energy is ultimately transferred to the e lectrons. Here we propose that Fermi acceleration is this mechanism, t he process by which the electrons are energized and by which the casca ding MHD turbulence is dissipated. We point out that in the expected c ascade MHD fluctuations of scale 1 km can Fermi-accelerate electrons f rom 0.1 keV to similar to 25 keV on the subsecond timescales observed in impulsive flares, provided there is sufficient trapping and scatter ing of electrons in the MHD turbulence. We show that these same fluctu ations provide the required trapping; they confine the electrons withi n the turbulent region until the turbulence is dissipated. This result s in the energization of all of the electrons in each large-scale (5 x 10(7) cm) turbulent eddy to 25 keV. The Fermi process also requires e fficient scattering so that the pitch-angle distribution of the accele rating electrons remains isotropic. We propose that the electrons unde rgo resonant scattering by high-frequency plasma R-waves that, as sugg ested by others (Hamilton and Petrosian 1992), are generated by the re connection. Ions are not scattered by R-waves. Provided that there is negligible generation of ion-scattering plasma turbulence (e.g., L-wav es) by the reconnection or the MHD turbulence, the ions will not Fermi -accelerate and the cascading energy is transferred only to the electr ons. We conclude that, given this situation, electron Fermi accelerati on can plausibly account for the electron bulk energization in impulsi ve solar flares.