MHD MODELING OF MAGNETOTAIL INSTABILITY FOR LOCALIZED RESISTIVITY

We present results of a three-dimensional MHD simulation of magnetotail evolution initiated by a sudden occurrence or increase of spatially localized resistivity as the major expected consequence of some localized microinstability. Because of the absense of a quantitative model, possible variations of resistivity levels with current density, or the reduction thereof, are not incorporated in the present investigation. The emphasis of the study is on an investigation of the changes to the overall evolution brought about by this localization, in particular, on the disruption and diversion of the cross-tail current and the nonlinear evolution of the magnetotail instability. The immediate consequences of the occurrence of the localized resistance and the resulting electric field are a reduction and diversion of the electric current around the region of high resistivity, associated with an increase of B(z) (''dipolarization'') at the earthward edge and a decrease of B(z) at the tailward edge of this region. These effects, however, are localized and do not involve a reduction of the total cross-tail current and hence do not lead to the global development of a ''substorm current wedge,'' which includes not only the reduction of the cross-tail current but also the buildup of a global field-aligned current system of ''region 1'' type (toward the Earth on the dawnside and away on the duskside of the tail). Such signatures develop at a later time, as consequences of a three-dimensional tearing instability, which is triggered by the occurrence of the resistivity. These features are found in combination with plasmoid formation and ejection, quite similar to results of earlier simulations with uniform resistivity. Differences are found in the timescale of the evolution, which tends to be shorter for localized resistivity, and in the propagation of the dipolarization effects in the equatorial plane. Whereas for uniform resistivity the temporal increase in B(z) tends to propagate tailward, apparently due to a pileup effect in the near-Earth region, an earthward propagation is found for the localized resistivity. This propagation results from an earthward convection of the gradient in B(z), which is set up by the reconnection process further tailward.