Plasma density enhancements in the high‐altitude polar cap region observed on Akebono

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The plasma density in the polar cap ionosphere is generally low (<103 cm−3 above 3000 km), mainly because of plasma escape from the ionosphere along open magnetic‐field lines. The Akebono satellite occasionally encounters regions of unusually high plasma density (≥103 cm−3) above 4000 km altitude, in which the thermal plasma exhibits a distinctively low electron temperature (<3000 K) and low parallel ion drift velocity (≤1 km/s). Such events are almost always observed on the dusk side. The occurrence of low electron temperature and ion drift velocity appears to suggest the antisunward convection of high‐density plasma into the polar cap, and the decrease in electron temperature due to the disruption of field‐aligned heat flux in the high‐altitude polar cap.