Fast impulsive reconnection and current sheet intensification due to electron pressure gradients in semi-collisional plasmas

A numerical simulation of forced reconnection and current sheet growth due to inward boundary flows in semi-collisional plasmas is presented, and contrasted with the results of an incompressible resistive MHD simulation in the high-Lundquist-number regime. Due to the presence of electron pressure (or Hall currents) in the generalized Ohm's law, the reconnection dynamics makes an impulsive transition from a slow linear regime to a nonlinear regime characterized by fast reconnection and current sheet intensification at a near-Alfvénic rate. The current sheet spanning Y-points in the early nonlinear regime shrinks and approaches an X-point geometry. The spatial scale of the collisionless parallel electric field is the ion skin depth, and decoupled from the spatial scale of the parallel current which is much narrower and determined by the Lundquist number.