Role of Electron Inertia and Reconnection Dynamics in a Stressed X-point Collapse with a Guide-Field

TL;DR

This study uses relativistic PIC simulations to explore how electron inertia influences magnetic reconnection at an X-point with a guide-field, revealing a new reconnection regime with vortex formation and a burst of electric field.

Contribution

It demonstrates the dominant role of electron inertial terms in reconnection dynamics with a strong guide-field, extending understanding from 2D to 3D configurations.

Findings

Electron inertial effects dominate at the X-point with strong guide-fields.

A new reconnection regime with magnetic island formation and vortex flows emerges.

Vortex tube tilt angles match theoretical predictions.

Abstract

In previous simulations of collisionless 2D magnetic reconnection it was consistently found that the term in the generalised Ohm's law that breaks the frozen-in condition is the divergence of the electron pressure tensor's non-gyrotropic components. A fully relativistic particle-in-cell (PIC) code was used to model $X$-point collapse with a guide-field in two and three spatial dimensions. We show that in a 2D $X$-point collapse with a guide-field close to the strength of the in-plane field, the increased induced shear flows along the diffusion region lead to a new reconnection regime in which electron inertial terms play a dominant role at the $X$-point. This transition is marked by the emergence of a magnetic island - and hence a second reconnection site - as well as electron flow vortices moving along the current sheet. The reconnection electric field at the $X$-point is shown to exceed all lower guide-field cases for a brief period, indicating a strong burst in reconnection. By extending the simulation to three spatial dimensions it is shown that the locations of vortices along the current sheet (visualised by their $Q$-value) vary in the out-of-plane direction, producing tilted vortex tubes. The vortex tubes on opposite sides of the diffusion region are tilted in opposite directions, similarly to bifurcated current sheets in oblique tearing-mode reconnection. The tilt angles of vortex tubes were compared to a theoretical estimation and were found to be a good match. Particle velocity distribution functions for different guide-field runs, for 2.5D and 3D simulations, are analysed and compared.