Resistive magnetohydrodynamic simulations of X-line retreat during magnetic reconnection

TL;DR

This study uses resistive MHD simulations to analyze how the motion of current sheets influences magnetic reconnection, revealing that X-line retreat involves complex plasma flows and can be described by a derived rate expression.

Contribution

It introduces a detailed simulation of dual reconnection sites and derives an expression for X-line retreat rate considering advection and magnetic diffusion effects.

Findings

Reconnection rate is higher with two moving sites than a single site.

Unobstructed outflows are faster and longer than obstructed ones.

X-line retreat speed ranges from ~0.02 to 0.07 times the upstream Alfven velocity.

Abstract

To investigate the impact of current sheet motion on the reconnection process, we perform resistive magnetohydrodynamic (MHD) simulations of two closely located reconnection sites which move apart from each other as reconnection develops. This simulation develops less quickly than an otherwise equivalent single perturbation simulation but eventually exhibits a higher reconnection rate. The unobstructed outflow jets are faster and longer than the outflow jets directed towards the magnetic island that forms between the two current sheets. The X-line and flow stagnation point are located near the trailing end of each current sheet very close to the obstructed exit. The speed of X-line retreat ranges from ~0.02-0.06 while the speed of stagnation point retreat ranges from ~0.03-0.07, in units of the initial upstream Alfven velocity. Early in time, the flow stagnation point is located closer to the center of the current sheet than the X-line, but later on the relative positions of these two points switch. Consequently, late in time there is significant plasma flow across the X-line in the opposite direction of X-line retreat. Throughout the simulation, the velocity at the X-line does not equal the velocity of the X-line. Motivated by these results, an expression for the rate of X-line retreat is derived in terms of local parameters at the X-point. This expression shows that X-line retreat is due to both advection by the bulk plasma flow and diffusion of the normal component of the magnetic field.