Storm-time electron density enhancement in the cleft ion fountain

To determine the characteristics and origin of observed storm-time electron density enhancements in the polar cap, and to investigate the spatial extent (noon-midnight direction) of associated O⁺ ion outflows, we analyzed nearly simultaneous observations of such electron density enhancements from the Akebono satellite and ion upflows from the Polar satellite during a geomagnetic storm occurring on 6 April 2000. The Akebono satellite observed substantial electron density enhancements by a factor of ~10-90 with a long duration of ~15 h at ~2 R _E in the southern polar region. The Polar satellite outflow measurements in the northern polar cap at ~7-4 R _E exhibited velocity filtering of the ~100 eV to ~0 eV (from the spacecraft potential) ion outflow from the cleft ion fountain, with resultant temperatures declining from ~3 eV to 0.03 eV with increasing distance from the cusp. Similar velocity filtering was detected in the southern polar cap at ~1.8-3.5 R _E. The region of O⁺ ion outflows with fluxes exceeding 5 × 10⁸ /cm²/s (mapped to 1000 km altitude) extended ~10° MLAT (~1000 km) at the ionosphere from the cusp/cleft into the dayside polar cap at ~2.5 R _E. These coordinated Akebono-Polar observations are consistent with the development of storm-time electron density enhancements in the polar cap as a result of the bulk outflow of low-energy plasma as part of the cleft ion fountain. The large spatial scale, large ion fluxes, and the long duration indicate significant supply of very-low-energy O⁺ ions to the magnetosphere through this region.