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Ann. Geophys., 20, 1743-1750, 2002
www.ann-geophys.net/20/1743/2002/
© European Geosciences Union 2002
Altitude dependence of plasma density in the auroral zone
P. Janhunen1, A. Olsson2, and H. Laakso3
1Finnish Meteorological Institute, Geophysical Research, Helsinki, Finland
2Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden
3ESTEC, Space Science Department, Noordwijk, The Netherlands
Correspondence to: P. Janhunen (Pekka.janhunen@fmi.fi)
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 RE
radial distance, where RE 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
RE radial distance, separate cavities above 4 RE
occur in the midnight sector for all Kp and also in the
morning sector for high Kp. 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)
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