Substorm evolution as revealed by THEMIS satellites and a global MHD simulation

The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near-Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near-Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.