Mesoscale dayside convection vortices and their relation to substorm phase

Measurements made with the first two pairs of the northern hemisphere component of the Super Dual Auroral Radar Network (SuperDARN) have revealed the intermittent existence of a mesoscale convection vortex in the high-latitude postnoon ionosphere. The vortex is a feature of the substorm growth phase and is typically centered between 1430 and 1530 MLT and 75 degrees and 80 degrees invariant latitude. It has a diameter ranging from a few hundred to similar to 1000 km and an associated potential drop of 5-10 kV. The vortex is centered on a filamentary upward field-aligned current with an estimated magnitude approaching 3 mu A/m(2). The vortex is centered near the sunward end of the dusk convection cell just poleward of the sunward convecting plasma and just duskward of the region where the sunward convecting plasma rotates sharply poleward and enters the polar cap. As the plasma convects poleward, it passes through an irregularity zone that has been associated with the ionospheric, footprint of the cusp. A remarkable feature of the vortex is that it disappears concurrently with the onset of a substorm expansion phase in the midnight sector. Several magnetospheric source mechanisms, including the Kelvin-Helmholtz and tearing mode instabilities, flux transfer events, and macroscale current systems, have been considered for the vortex. The best explanation appears to be that the vortices are associated with filamentary field-aligned currents that are driven by the cross polar cap potential and close as Pedersen currents through the cusp region. The disappearance of the vortex following the onset of an expansion phase is attributed to a redirection of magnetospheric closure currents as a consequence of the significant increase in nightside conductivity during a substorm expansion.