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.