Storm‐time electron density enhancement in the cleft ion fountain

Abstract

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 RE in the southern polar region. The Polar satellite outflow measurements in the northern polar cap at ∼7–4 RE 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 RE. 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 RE. 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.