Three-dimensional X-line spreading in asymmetric magnetic reconnection

Using large-scale 3-D particle-in-cell simulations, we investigate the spreading of the X-line out of the reconnection plane under a strong guide field in asymmetric reconnection. The X-line spreading speed depends strongly on the equilibrium current sheet thickness.

In a simulation with a thick, ion-scale equilibrium current sheet (CS), the X-line spreads at the ambient species drift speeds, which are significantly lower than the Alfven speed based on the guide field V-Ag(sub-Alfvenic spreading). In simulations with a sub-ion-scale CS, the X-line spreads at V-Ag instead (Alfvenic spreading). An Alfvenic signal consistent with kinetic Alfven waves develops and propagates, leading to CS thinning and extending, which ultimately causes reconnection onset.