Reconnection Onset in Overstretched Thin Current Sheets: 3‐D Kinetic Picture

Abstract Evolution of the magnetotail equilibrium, whose current density iso‐contours are stretched beyond the magnetic field lines due to small ion anisotropy and agyrotropic effects on the scales of the thermal ion gyroradius (the so‐called overstretched thin current sheet) is considered using 3‐D explicit particle‐in‐cell simulations. It is found that the slow driving of such an overstretched current sheet with a flux accumulation ridge (a localized along the tail region where the equatorial magnetic field is enhanced above its background value due to accumulated magnetic flux) causes rapid acceleration of the latter with fast differential (i.e., divergent in the system moving with the average bulk velocity of the species) plasma flows. The divergent flows drive the reconnection onset in a regime of the ion tearing instability with the dominant transfer of the energy from the electromagnetic field to ions. They are modestly, compared to similar processes in isotropic plasmas, disturbed by ballooning/interchange and flapping motions. The lower‐hybrid‐drift waves are excited at the edge of the plasma sheet, but in contrast to isotropic plasmas, they do not perturb the core current, which is localized deep inside the plasma sheet.