Flow braking and the substorm current wedge

Recent models of magnetotail activity have associated the braking of earthw ard flow with dipolarization and the reduction and diversion of cross-tail current, that is, the signatures of the substorm current wedge. Estimates o f the magnitude of the diverted current by Haerendel [1992] and Shiokawa et al. [1997, 1998] tend to be lower than results from computer simulations o f magnetotail reconnection and tail collapse [Birn and Hesse, 1996], despit e similar underlying models. An analysis of the differences between these e stimates on the basis of the simulations gives a more refined picture of th e diversion of perpendicular into parallel currents. The inertial currents considered by Haerendel [1992] and Shiokawa et al. [1997] contribute to the initial current reduction and diversion, but the dominant and more permane nt contribution stems from the pressure gradient terms, which change in con nection with the field collapse and distortion. The major effect results fo rm pressure gradients in the z direction, rather than from the azimuthal gr adients [Shiokawa et al., 1998], combined with changes in B-y and B-x. The reduction of the current density near the equatorial plane is associated wi th a reduction of the curvature drift which overcompensates changes of the magnetization current and of the gradient B drift current. In contrast to t he inertial current effects, the pressure gradient effects persist even aft er the burst of earthward flow ends.