Very High Temporal Resolution Measurements of Subauroral Electric fields Associated With Bursts of Pi 1 Pulsations During the Substorm Expansion Phase

Recent innovations in Doppler analysis techniques used by the SuperDARN radars have allowed the temporal resolution of plasma velocity and electric field measurements to be improved by more than an order of magnitude. Tests of this new capability were made using the Wallops Island SuperDARN radar during the post- midnight hours of August 1, 2007. This was a geomagnetically-disturbed period in which a number of substorm expansions occurred. During one of these expansions, the Wallops radar observed a burst of Pi 1 electric-field pulsations with periods in the vicinity of 14 seconds. These electric-field pulsations were observed in the subauroral ionosphere in the vicinity of the plasmapause at 02 MLT. Nearby ground-based magnetometers observed related bursts of Pi 1 magnetic pulsations with the same period. Longer period Pi 2 pulsations were also observed in the magnetic data but were not observed in the electric field measurements suggesting that the currents producing the longer period magnetic pulsations were not local to the overhead ionosphere. Measurements from auroral zone magnetometers in central Canada show similar bursts of Pi 1 and Pi 2 pulsation over central Canada in response to a substorm expansion phase. The Pi 1 pulsations appear to be propagating away from midnight and show evidence of both toroidal and poloidal modes. They are thought to be excited by the injection of fresh plasma into the inner magnetosphere in response to dipolarization of magnetotail field lines. These observations show the value of having electric field data as well as magnetic field data in interpreting short-period MHD wave phenomena.