Relativistic magnetic reconnection in collisionless ion-electron plasmas explored with particle-in-cell simulations

TL;DR

This study uses 2D particle-in-cell simulations to explore relativistic magnetic reconnection in collisionless ion-electron plasmas, revealing unique energy conversion mechanisms and flow properties relevant to high-energy astrophysical phenomena.

Contribution

First-principles PIC simulations of relativistic ion-electron plasmas uncover novel reconnection dynamics and electric field sustenance mechanisms at high magnetizations.

Findings

Reconnection outflows are dominated by thermal agitation at high magnetizations.

Reconnection electric field is sustained more by bulk inertia than thermal inertia at high electron magnetization.

Reconnection rates are slightly higher than in non-relativistic cases, with normalized rates between 0.14 and 0.25.

Abstract

Magnetic reconnection is a leading mechanism for magnetic energy conversion and high-energy non-thermal particle production in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas (e.g., microquasars or AGNs) - a regime where first principle studies are scarce. We present 2D particle-in-cell (PIC) simulations of low $\beta$ ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic energy exceeding the plasma rest-mass energy. We identify outstanding properties: (i) For relativistic inflow magnetizations (here $10 < \sigma_e < 360$), the reconnection outflows are dominated by thermal agitation instead of bulk kinetic energy. (ii) At large inflow electron magnetization ($\sigma_e > 80$), the reconnection electric field is sustained more by bulk inertia than by thermal inertia. It challenges the thermal-inertia-paradigm and its implications. (iii) The inflows feature sharp transitions at the entrance of the diffusion zones. These are not shocks but results from particle ballistic motions, all bouncing at the same location, provided that the thermal velocity in the inflow is far smaller than the inflow E cross B bulk velocity. (iv) Island centers are magnetically isolated from the rest of the flow, and can present a density depletion at their center. (v) The reconnection rates are slightly larger than in non-relativistic studies. They are best normalized by the inflow relativistic Alfv\'en speed projected in the outflow direction, which then leads to rates in a close range (0.14-0.25) thus allowing for an easy estimation of the reconnection electric field.