THE SUBSTORM CURRENT WEDGE AND FIELD-ALIGNED CURRENTS IN MHD SIMULATIONS OF MAGNETOTAIL RECONNECTION

We present results of a three-dimensional MHD simulation of magnetotail reconnection, which, in contrast to earlier ones, starts from an equilibrium configuration that contains a region 1 type field-aligned current system at the plasma sheet/lobe boundary. This current system is found from equilibrium theory as the consequence of boundary conditions, which require that field lines at large distances from the Earth become aligned with the tail axis, consistent with observations and the expected consequences of the interaction with the solar wind. As in earlier simulations, the dynamic evolution develops from a slow diffusion to the fast growth of a three-dimensional tearing mode, initiated by the sudden occurrence or increase of resistivity, leading again to the formation and subsequent tailward ejection of a plasmoid, associated with fast plasma flows. In addition, we find changes of the electric current system which are consistent with the substorm current wedge picture inferred from observations, i.e., a deviation of cross-tail current from a near-Earth tail region toward the Earth on the dawn side and away on the dusk side, associated with filed-aligned currents of the same signature. While the total cross-tail is reduced at the location of the near-Earth reconnection site, the local current density at the near-Earth X line actually increased, consistent with the strong plasma sheet thinning in that region. The source regions of the field-aligned currents, indicated by strong peaks of DELTA.j parallel-to, are identified as regions of strong plasma flow vorticity near the boundaries of the fast earthward flow earthward of the reconnection site. This flow shear increases preexisting magnetic shear resulting in an increased twist of magnetic flux tubes connected with these regions and thus a generation or increase of the field-aligned currents. We note that the current diversion in our simulation is not caused by a locally enhanced resistivity (although in reality this might be the case) but by the localization of reconnection in a three-dimensional tail configuration.