Stochastic Fermi acceleration of subrelativistic electrons and its role in impulsive solar flares

Abstract

We re-examine stochastic Fermi acceleration (STFA) in the low-energy (Newtonian) regime in the context of solar flares. The particle energization rate depends on a dispersive term and on a coherent gain term. The energy dependence of pitch angle scattering is important for determining the electron energy spectrum. For scattering by whistler wave turbulence, STFA produces a quasi-thermal spectrum. A second well-constrained scattering mechanism is needed for STFA to match the observed 10-100 keV non-thermal spectrum. We suggest that STFA most plausibly acts as phase one of a two-phase particle acceleration engine in impulsive flares: STFA can match the thermal spectrum below 10 keV, and possibly the power-law spectrum between 10 and 100 keV, given the proper pitch angle scattering. However, a second phase, such as shock acceleration at loop tops, is probably required to match the spectrum above the observed knee at 100 keV. Understanding this knee, if it survives further observations, is tricky.