Steep plasma depletion in dayside polar cap during a CME-driven magnetic storm

A series of steep plasma depletions was observed in the dayside polar cap during an interval of highly enhanced electron density on 14 October 2000 through EISCAT Svalbard Radar (ESR) field‐aligned measurements and northward‐directed low‐elevation measurements. Each depletion started with a steep dropoff to as low as 1011 m− 3 from the enhanced level of ∼3 × 1012 m−3 at F2 region altitudes, and it continued for 10–15 min before returning to the enhanced level. These depletions moved poleward at a speed consistent with the observed ion drift velocity. DMSP spacecraft observations over an extended period of time which includes the interval of these events indicate that a region of high ion densities extended into the polar cap from the equatorward side of the cusp, i.e., a tongue of ionization existed, and that the ion densities were very low on its prenoon side. Solar wind observations show that a sharp change from IMF By > 0 to By