Space Physics Seminar winter-2016

Is the pressure gradient a driver of the substorm current wedge?

Substorm current wedge (SCW) formation is believed to be related to the flow braking and diversion process. In this work, good temporal and spatial correlations are found between earthward flows during five THEMIS tail seasons and substorm onsets identified using the midlatitude positive bay index. Flow occurrence is found to peak at substorm onset. More than half the flows observed within one hour of substorm onsets occur within ten minutes of onsets. In addition, Most of these flows (85%) are found inside an SCW between its upward and downward field-aligned currents (FACs). It has been suggested that these FACs are generated either by flow vortices, pressure gradient, or both. It is shown that the flow speed (related to the flow vortices) decays quickly within several minutes. On the other hand, the equatorial thermal pressure (related to the pressure gradient) increases and persists for about an hour, and has a trend similar to that for the westward electrojet and FACs of the SCW. Therefore, the SCW is likely sustained by the pressure gradient rather than short-lived flow vortices. The pressure gradient, calculated when three THEMIS probes were distributed in a triangular configuration in the equatorial plane, was found to be well organized relative to the central meridian (CM) of the SCW. The component ?P_x increases for all substorms; while ?P_y increases in magnitude and points toward the center of the current wedge. The non-alignment of ?P and ?V should generate an SCW with a quadrupole FAC pattern, similar to that seen in global MHD and RCM-E simulations. In these simulations the inner current loop is weaker than the outer loop so that the magnetic effect at geosynchronous orbit and on the ground is that of the outer loop diminished in strength by the inner loop, which resembles a classic SCW.