On the formation of the high-altitude stagnant cusp: Cluster observations

On 16 March 2002, Cluster moved from nightside to dayside, across the high-altitude northern cusp during an extended period of relatively steady positive IMF BY and B-Z. Combined Cluster and SuperDARN data imply the existence of two reconnection sites: in the high-latitude northern hemisphere dusk and southern hemisphere dawn sectors. Within the cusp, Cluster encounters 3 distinct plasma regions. First, injections of magnetosheath-like plasma associated with dawnward and sunward convection suggest Cluster crosses newly-reconnected field lines related to the dusk reconnection site. Second, Cluster observes a Stagnant Exterior Cusp ( SEC), characterized by nearly isotropic and stagnant plasma. Finally, Cluster crosses a region with significant antifield-aligned flows. We suggest the observed SEC may be located on newly re-closed field lines, reconnected first poleward of the northern hemisphere cusp and later reconnected again poleward of the southern hemisphere cusp. We discuss how the Cluster observations correspond to expectations of 'double reconnection' model.