Details of current disruption and diversion in simulations of magnetotail dynamics

Properties of current disruption and diversion are investigated on the basis of resistive MHD simulations which include the inner tail region with the transition toward a dipole field and the dynamic formation of a thin current sheet in the inner tail. As in earlier simulations, magnetotail instability leads to reconnection and plasmoid formation and ejection, accompanied by features of the substorm current wedge. In contrast to earlier simulations of the breakup of a thick current sheet, the presence of the thin current sheet leads to a much faster evolution on the timescale of few minutes and to larger electric fields (up to 20 mV/m), concentrated mostly in the dipolarizing region earthward of the reconnection site. The fast growth is relatively insensitive to the resistivity model used. The current “disruption” is largely a consequence of an expansion of the near‐Earth current sheet, associated with the dipolarization. It starts well earthward of the reconnection site in association with a pileup of magnetic flux and propagates tailward. Field‐aligned currents of region 1 type (toward the Earth on the dawnside and away on the duskside) at the near‐Earth boundary are found to increase both during the driven phase of the thin current sheet formation and during the energy release and current disruption phase. They result from a diversion of perpendicular into parallel currents, which is associated with a gradient of the magnetic flux tube volume directed away from midnight at the flank edges of the dipolarization region. Locally, the current diversion takes place earthward of the reconnection site, predominantly in shear layers above and below the neutral sheet rather than close to the neutral sheet. The peaks of the field‐aligned current densities associated with the substorm wedge do not follow field lines very closely, because of the presence of strong perpendicular currents, so that care must be taken in using these features for a mapping between the Earth and the equatorial plane.