Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection

TL;DR

This study investigates how self-generated turbulence influences non-Maxwellian electron velocity distributions during collisionless guide-field magnetic reconnection, revealing anisotropic EVDFs, electron beams, and turbulence effects across varying guide-field strengths.

Contribution

It provides the first detailed analysis of EVDFs and turbulence feedback in oblique reconnection with strong guide-fields using fully-kinetic simulations.

Findings

EVDFs are anisotropic with electron beams propagating along separatrices.

Self-generated turbulence causes non-linear electron acceleration and phase space holes.

Guide-field strength affects electron-beam drift speed and instability properties.

Abstract

Non-Maxwellian electron velocity space distribution functions (EVDF) are useful signatures of plasma conditions and non-local consequences of collisionless magnetic reconnection. In the past, EVDFs were obtained mainly for antiparallel reconnection and under the influence of weak guide-fields in the direction perpendicular to the reconnection plane. EVDFs are, however, not well known, yet, for oblique (or component-) reconnection in dependence on stronger guide-magnetic fields and for the exhaust (outflow) region of reconnection away from the diffusion region. In view of the multi-spacecraft Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of collisionless magnetic reconnection in dependence on the guide-field strength $b_g$ from small ($b_g\approx0$) to very strong ($b_g=8$) guide-fields, taking into account the feedback of the self-generated turbulence. For this sake, we carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM code. We obtained anisotropic EVDFs and electron beams propagating along the separatrices as well as in the exhaust region of reconnection. The beams are anisotropic with a higher temperature in the direction perpendicular rather than parallel to the local magnetic field. The beams propagate in the direction opposite to the background electrons and cause instabilities. We also obtained the guide-field dependence of the relative electron-beam drift speed, threshold and properties of the resulting streaming instabilities including the strongly non-linear saturation of the self-generated plasma turbulence. This turbulence and its non-linear feedback cause non-adiabatic parallel electron acceleration and EVDFs well beyond the limits of the quasi-linear approximation, producing phase space holes and an isotropizing pitch-angle scattering.