MAGNETOHYDRODYNAMIC TURBULENCE DISSIPATION AND STOCHASTIC PROTON ACCELERATION IN SOLAR-FLARES

Two MHD modes, Alfven and fast magnetosonic waves, are capable of stochastically accelerating protons from super-Alfvenic to ultrarelativistic energies in solar flares. Acceleration occurs through the Landau resonance for fast magnetosonic waves, and through the cyclotron resonance for Alfven waves. The Landau resonance, however, is also satisfied by thermal electrons in a flare plasma, resulting in most of the magnetosonic wave energy being dissipated on electron heating rather than on stochastic proton acceleration. In contrast, thermal electrons cannot satisfy the cyclotron resonance, leaving stochastic gyroresonant proton acceleration as the dominant linear wave-particle interaction for Alfven waves. Alfven waves are also subject to a nonlinear wave-particle interaction as well. Specifically, nonlinear Landau damping is an important dissipation process for Alfven waves under solar flare conditions. Moreover, nonlinear Landau damping efficiently and selectively heats the ambient protons and can thus essentially preaccelerate a significant number of them out of a low-temperature distribution to super-Alfvenic speeds, where they can be subsequently accelerated to ultrarelativistic energies by gyroresonant interactions. A spectrum of Alfven waves, therefore, can energize protons from low-temperature thermal to ultrarelativistic energies through a combination of nonlinear and linear wave-particle interactions. Furthermore, other dissipation processes are unimportant under solar flare conditions, leaving proton heating and acceleration as the dominant damping mechanisms for Alfven waves.