Electron acceleration by turbulent plasmoid reconnection

TL;DR

This paper investigates how turbulent plasmoid reconnection accelerates electrons in space plasmas, revealing that turbulence-induced EMFs near X-points can produce power-law energy spectra up to 60 keV.

Contribution

It introduces a mean-field turbulence model integrated with MHD simulations to study electron acceleration in turbulent reconnection, a novel approach for this problem.

Findings

Localized EMFs near X-points efficiently accelerate electrons.

Magnetic-field-aligned EMFs dominate the acceleration process.

Electrons can reach energies up to 60 keV in Hermean magnetotail conditions.

Abstract

In space and astrophysical plasmas, like in planetary magnetospheres, as that of Mercury,energetic electrons are often found near current sheets (CSs), which hints at electron acceleration by magnetic reconnection. Unfortunately, electron acceleration by reconnection is not well understood, yet. In particular, acceleration by turbulent plasmoid reconnection. We have investigated electron acceleration by turbulent plasmoid reconnection, described by MHD simulations, via test particle calculations. In order to avoid resolving all relevant turbulence scales down to the dissipation scales, a mean-field turbulence model is used to describe the turbulence of sub-grid scales (SGS) and their effects via a turbulent electromotive force (EMF). The mean-field model describes the turbulent EMF as a function of the mean values of current density, vorticity, magnetic field as well as of the energy, cross-helicity and residual helicity of the turbulence. We found that, mainly around X-points of turbulent reconnection, strongly enhanced localized EMFs most efficiently accelerated electrons and caused the formation of power-law spectra. Magnetic-field-aligned EMFs, caused by the turbulence, dominate the electron acceleration process. Scaling the acceleration processes to parameters of the Hermean magnetotail, electron energies up to 60 keV can be reached by turbulent plasmoid reconnection through the thermal plasma.