Transient Polar-Cap Potential Enhancements During the April 2002 Storm Interval

The interval from 16-24 April 2002 exhibited a series of solar events that produced significant disturbances in the Earth's magnetosphere and ionosphere. One phenomenon that has been reported in association with these disturbances is the observation of rapid (sawtooth-like) enhancements in the energetic particle population at geostationary orbit that appear nearly concurrently at all local times. These enhancements are similar to the particle enhancements observed near midnight in association with magnetospheric substorms, but they do not exhibit the delayed response at other local times. The objective of this paper is to report the association of these events with transient enhancements in the cross polar cap potential drop. The observations that we present were obtained with the northern hemisphere component of the SuperDARN radar network. Specifically, we observed rapid increases (2-6 minute) in the cross-polar-cap potential that endured for 6-30 minutes immediately preceding the sawtooth onset. As the particle fluxes increase, the cross-polar-cap potential observed by SuperDARN drops rapidly to preexisting levels. The magnitudes of the increase in electrical potential-drop have been observed to range form 10-30 kV and they are observed to occur for a wide range of IMF conditions. Since similar enhancements in the cross-polar-cap potential drop are also observed at non-storm times, we suggest that the transient enhancements we observe may be associated with the substorm growth phase and that the sawtooth events during the April interval may be particularly strong substorm onsets.