A new convection state at substorm onset: Results from an MHD study

A global simulation of a December 22, 1996 substorm revealed a previously unknown state of magnetospheric convection. Earthward flow from a tail neutral line reversed direction in the inner magnetosphere and formed a large-scale vortical nightside convection pattern. The dawnside vortex formed a flux rope just prior to the substorm onset, but this was not the cause of the substorm breakup. The vortex structure of the dusk-side flow created strong field-aligned currents directed away from the Earth that connected to the region of maximum auroral luminosity in the ionosphere. These currents are probably related to the westward traveling surge. The flow reversal and formation of the tail vortices may be a result of the simulation's earthward convection being limited by the high near-Earth plasma pressure and/or by line-tying caused by the high ionospheric conductance.