Large-Amplitude ExB Plasma Drifts in the High-Latitude Ionosphere

We investigate the fastest ExB plasma drifts observed by the Vector Electric Field Instrument double probe experiment on the NASA's Dynamics Explorer-2 (DE-2) satellite. In the limited DE-2 database, we find 507 events where the zonal ExB velocity exceeds 4 km/s, in which for 91 events the velocity exceeds 6 km/s and 16 events where the drifts are greater than 8 km/s. One primary group of fast drifts consists of nightside events that occur mostly within the shadowed ionosphere and are related to auroral precipitation (e.g., inverted-V, sub-auroral ion drifts, Alfven waves) driven by magnetospheric processes. Another major group of fast drifts corresponds to dayside events that are often directly driven by the solar wind and can occur within either the sunlit or shadowed ionosphere. The fast drifts are observed at all DE-2 altitudes between 300 and 1,000 km with no altitude dependence on the drift velocity. They are generally observed at all MLT sectors although there appears to be a dawn-dusk asymmetry with the fewest events around 15 MLT. In contrast, the occurrence peaks between 6 and 12 MLT. The events occur in the auroral and sub-auroral altitudes, normally between 60 and 83° of invariant latitude, although in the pre-midnight sector, events are found as low as 50° of invariant latitude, corresponding to sub-auroral ion drift (SAID) events. Surprisingly, with the exception of SAID events, the fast drift events occur during all geomagnetic conditions with the velocities showing no dependence on geomagnetic activity.