Electron Acceleration from Expanding Magnetic Vortices During Reconnection with a Guide Field

Abstract

Magnetic reconnection is believed to be responsible for the acceleration of energetic electrons with a power-law spectrum in the solar wind and solar flares. However, recent studies of the leading mechanism of electron acceleration in reconnection, namely the acceleration by tearing instability induced multi-islands, demonstrates that this mechanism suffers from an “injection problem” for mildly relativistic reconnection acceleration. In this paper, we investigate a new type of reconnection acceleration in which an electron Kelvin–Helmholtz instability (EKHI) is driven as the current sheet reaches electron inertial length scales during magnetic reconnection with a strong guide field. Electrons are accelerated by stochastic electric fields, induced by the EKHI generated vortices that expand rapidly, and a power-law electron energy spectrum with index α  ∼ 3.5 is produced ( W is the electron kinetic energy and f ( W ) is the energy distribution function). We show that the mechanism is a second-order Fermi acceleration process, and the index where a  =  B g / B 0 , which is determined by the ratio of the spatial scale of the inductive electric field D to that of vortices R and the ratio of guide field B g to asymptotic magnetic field B 0 .