Nonthermally Dominated Electron Acceleration during Magnetic Reconnection in a Low-beta Plasma

By means of fully kinetic simulations, we investigate electron acceleration during magnetic reconnection in a nonrelativistic proton--electron plasma with conditions similar to solar corona and flares. We demonstrate that reconnection leads to a nonthermally dominated electron acceleration with a power-law energy distribution in the nonrelativistic low-$\beta$ regime but not in the high-$\beta$ regime, where $\beta$ is the ratio of the plasma thermal pressure and the magnetic pressure. The accelerated electrons contain most of the dissipated magnetic energy in the low-$\beta$ regime. A guiding-center current description is used to reveal the role of electron drift motions during the bulk nonthermal energization. We find that the main acceleration mechanism is a \textit{Fermi}-type acceleration accomplished by the particle curvature drift motion along the electric field induced by the reconnection outflows. Although the acceleration mechanism is similar for different plasma $\beta$, low-$\beta$ reconnection drives fast acceleration on Alfv\'enic timescales and develops power laws out of thermal distribution. The nonthermally dominated acceleration resulting from magnetic reconnection in low-$\beta$ plasma may have strong implications for the highly efficient electron acceleration in solar flares and other astrophysical systems.