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Altitude dependence of plasma density in the auroral zone

Abstract. We study the altitude dependence of plasma depletions above the auroral region in the 5000–30 000 km altitude range using five years of Polar spacecraft potential data. We find that besides a general decrease of plasma density with altitude, there frequently exist additional density depletions at 2–4 R E radial distance, where R E is the Earth radius. The position of the depletions tends to move to higher altitude when the ionospheric footpoint is sunlit as compared to darkness. Apart from these cavities at 2–4 R E radial distance, separate cavities above 4 R E occur in the midnight sector for all K p and also in the morning sector for high K p . In the evening sector our data remain inconclusive in this respect. This holds for the ILAT range 68–74. These additional depletions may be substorm-related. Our study shows that auroral phenomena modify the plasma density in the auroral region in such a way that a nontrivial and interesting altitude variation results, which reflects the nature of the auroral acceleration processes. Key words. Magnetospheric physics (auroral phenomena; magnetosphere–ionosphere interactions)