Orientation and stability of asymmetric magnetic reconnection x-line

TL;DR

This study uses 3D particle-in-cell simulations to explore how the orientation and stability of asymmetric magnetic reconnection x-lines develop, revealing that the x-line prefers an optimal orientation for maximum reconnection rate and can become turbulent when forced away from this orientation.

Contribution

It demonstrates that the x-line in asymmetric reconnection naturally adopts an optimal orientation for stability and reconnection efficiency, and shows how external forcing can induce oblique tearing modes and turbulence.

Findings

X-line develops along an optimal orientation for maximum reconnection rate

Non-gyrotropic pressure tensor divergence breaks the frozen-in condition

Forcing the x-line orientation leads to oblique tearing modes and turbulence

Abstract

The orientation and stability of the reconnection x-line in asymmetric geometry is studied using three-dimensional (3D) particle-in-cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x-line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2D simulations indicate that this x-line orientation maximizes the reconnection rate. The divergence of the non-gyrotropic pressure tensor breaks the frozen-in condition, consistent with its 2D counterpart. We then design 3D simulations with one dimension being short to fix the x-line orientation, but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. The development of oblique structure easily leads to turbulence inside small periodic systems.