Fast acceleration of transrelativistic electrons in astrophysical turbulence

Abstract

Highly energetic, relativistic electrons are commonly present in many astrophysical systems, from solar flares to the intra-cluster medium, as indicated by observed electromagnetic radiation. However, open questions remain about the mechanisms responsible for their acceleration, and possible re-acceleration. Ubiquitous plasma turbulence is one of the possible universal mechanisms. We study the energization of transrelativistic electrons in turbulence using hybrid particle-in-cell, which provide a realistic model of Alfv\'{e}nic turbulence from MHD to sub-ion scales, and test particle simulations for electrons. We find that, depending on the electron initial energy and turbulence strength, electrons may undergo a fast and efficient phase of energization due to the magnetic curvature drift during the time they are trapped in dynamic magnetic structures. In addition, electrons are accelerated stochastically which is a slower process that yields lower maximum energies. The combined effect of these two processes determines the overall electron acceleration. With appropriate turbulence parameters, we find that superthermal electrons can be accelerated up to relativistic energies. For example, with heliospheric parameters and a relatively high turbulence level, rapid energization to MeV energies is possible.