Guide Field Dependence of 3D X-Line Spreading During Collisionless Magnetic Reconnection

TL;DR

This study investigates how the presence and strength of a guide magnetic field influence the spreading mechanisms of 3D magnetic reconnection X-lines, revealing a transition from current-carrier-driven to Alfvén wave-driven spreading.

Contribution

It identifies the different mechanisms of X-line spreading under varying guide field strengths and predicts the transition point, verified through 3D simulations.

Findings

Weak guide fields cause unidirectional spreading via current carriers.

Strong guide fields induce bi-directional spreading through Alfvén waves.

A critical guide field strength marks the transition between mechanisms.

Abstract

Theoretical arguments and large-scale two-fluid simulations are used to study the spreading of reconnection X-lines localized in the direction of the current as a func- tion of the strength of the out-of-plane (guide) magnetic field. It is found that the mech- anism causing the spreading is different for weak and strong guide fields. In the weak guide field limit, spreading is due to the motion of the current carriers, as has been pre- viously established. However, spreading for strong guide fields is bi-directional and is due to the excitation of Alfv\'en waves along the guide field. In general, we suggest that the X-line spreads bi-directionally with a speed governed by the faster of the two mecha- nisms for each direction. A prediction on the strength of the guide field at which the spread- ing mechanism changes is formulated and verified with three-dimensional simulations. Solar, magnetospheric, and laboratory applications are discussed.