The evolution of fast reconnection in a three-dimensional current sheet system

Abstract

By showing the details of the numerical procedure, global dynamics of the current sheet system is systematically studied by three-dimensional magnetohydrodynamic simulations in the parameter range where the numerical resistivity is much smaller than the physical resistivity. In the absence of resistivity, initiated by a reconnection disturbance, current sheet thinning drastically occurs because of the sheet pinch, leading to extreme increase in current density around the X neutral point. For the uniform resistivity model, the drastic current increase is suppressed by the magnetic diffusion (reconnection), but the reconnection jet cannot be accelerated effectively, so that any fast reconnection mechanism cannot evolve; for the smaller resistivity, the current density at the X point becomes larger. Once current-driven anomalous resistivities build up, both the reconnection flow and the anomalous resistivity simultaneously grow to enhance each other, eventually giving rise to the Alfvénic fast reconnection jet. However, if the current sheet width is smaller than three times its thickness, the fast reconnection mechanism cannot be realized even in the presence of anomalous resistivity. Hence, only when a thin current sheet of sufficiently large scale is formed in space plasmas, the fast reconnection mechanism is likely to evolve drastically, leading to distinct plasma processes responsible for flares and substorms.