Power-law formation of nonthermal electrons in 3D low-beta reconnection

Abstract:

Magnetic reconnection is a promising mechanism to explain particle acceleration in solar flares and Earth's magnetotail. However, previous kinetic simulations have had difficulties obtaining a power-law energy spectrum, a common observational feature of the electron distributions. Using 3D fully kinetic particle-in-cell simulations of reconnection in the nonrelativistic low-beta regime, we show that a power-law energy spectrum can form and sustain extensively during the simulations. Compared with 2D simulations, where magnetic islands tend to trap high-energy electrons, 3D simulations enable these electrons to access the regions with stronger acceleration when they follow chaotic magnetic field lines and experience pitch-angle scattering by self-generated turbulence. These effects lead to a nearly constant acceleration rate for particles at different energies, as often assumed for a Fermi-type mechanism. The power-law index is a dynamical balance of particle acceleration and particle escape to the large flux rope. This study clarifies the formation condition of a power-law energy spectrum in a reconnection layer and has important implications for understanding particle energization during magnetic reconnection.