Effects of the April 2002 magnetic storm on density gradients in the middle and low latitude ionosphere

The redistribution of plasma in geospace at the onset of the April 2002 magnetic storm provides a remarkable example of storm-time forcing of the ionosphere and plasmasphere. The expansion of high-latitude electric fields to middle and low latitudes is accompanied by large-scale neutral density waves and winds that all act to rapidly modify the low latitude ionosphere. Earth's plasmasphere also responds dramatically to the enhanced plasmasheet flows and polarization electric fields formed in the dusk sector, with documented impulsive variability in the plasmaspheric boundary. A most important effect of rapid redistribution of plasma is to sharpen gradients in some regions, supress gradients in other, and introduce new gradient-producing processes. Here we investigate these developments at the onset of the storm using both space and ground-based assets.