Guide field dependence of 3-D X-line spreading during collisionless magnetic reconnection

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 function of the strength of the out-of-plane (guide) magnetic field. It is found that the mechanism 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 previously established. However, spreading for strong guide fields is bidirectional and is due to the excitation of Alfvén waves along the guide field. In general, we suggest that the X-line spreads bidirectionally with a speed governed by the faster of the two mechanisms for each direction. A prediction on the strength of the guide field at which the spreading mechanism changes is formulated and verified with three-dimensional simulations. Solar, magnetospheric, and laboratory applications are discussed. ©2012. American Geophysical Union. All Rights Reserved.