Effective resistivity in relativistic collisionless plasmoid-mediated reconnection

TL;DR

This paper uses kinetic simulations to analyze relativistic collisionless plasmoid-mediated magnetic reconnection, revealing the dominant role of nongyrotropic thermal pressure gradients in driving the non-ideal electric field and proposing a localized effective resistivity model.

Contribution

It introduces the first detailed analysis of the mechanisms behind non-ideal electric fields in relativistic plasmoid reconnection and proposes a kinetic physics-based effective resistivity model.

Findings

Non-ideal electric field mainly driven by nongyrotropic thermal pressure gradients.

Localized effective resistivity is significant only at X-points.

Proposed resistivity model captures key features of collisionless reconnection.

Abstract

Magnetic reconnection can power spectacular high-energy astrophysical phenomena by producing non-thermal energy distributions in highly magnetized regions around compact objects. By means of two-dimensional fully kinetic particle-in-cell (PIC) simulations we investigate relativistic collisionless plasmoid-mediated reconnection in magnetically dominated pair plasmas with and without guide field. In X-points, where diverging flows result in a non-diagonal thermal pressure tensor, a finite residence time for particles gives rise to a localized collisionless effective resistivity. Here, for the first time for relativistic reconnection in a fully developed plasmoid chain we identify the mechanisms driving the non-ideal electric field using a full Ohm's law by means of a statistical analysis based on our PIC simulations. We show that the non-ideal electric field is predominantly driven by gradients of nongyrotropic thermal pressures. We propose a kinetic physics motivated non-uniform effective resistivity model, which is negligible on global scales and becomes significant only locally in X-points, that captures the properties of collisionless reconnection with the aim of mimicking its essentials in non-ideal magnetohydrodynamic descriptions. This effective resistivity model provides a viable opportunity to design physically grounded global models for reconnection-powered high-energy emission.