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Forty-five months of continuous spacecraft potential measurements from the Polar satellite are used to study the average electron density in the magnetosphere and its dependence on geomagnetic activity and season. These measurements offer a straightforward, passive method for monitoring the total electron density in the magnetosphere, with high time resolution and a density range that covers many orders of magnitude. Within its polar orbit with geocentric perigee and apogee of 1.8 and 9.0 RE, respectively, Polar encounters a number of key plasma regions of the magnetosphere, such as the polar cap, cusp, plasmapause, and auroral zone that are clearly identified in the statistical averages presented here. The polar cap density behaves quite systematically with season. At low distance (~2 RE), the density is an order of magnitude higher in summer than in winter; at high distance (>4 RE), the variation is somewhat smaller. Along a magnetic field line the density declines between these two altitudes by a factor of 10–20 in winter and by a factor of 200–1000 in summer. A likely explanation for the large gradient in the summer is a high density of heavy ions that are gravitationally bound in the low-altitude polar cap. The geomagnetic effects are also significant in the polar cap, with the average density being an order of magnitude larger for high Kp; for an individual case, the polar cap density may increase even more dramatically. The plasma density in the cusp is controlled primarily by the solar wind variables, but nevertheless, they can be characterized to some extent in terms of the Kp index. We also investigate the local time variation of the average density at the geosynchronous distance that appears to be in accordance with previous geostationary observations. The average density decreases with increasing Kp at all MLT sectors, except at 14–17 MLT, where the average density remains constant. At all MLT sectors the range of the density varies by more than 3 orders of magnitude, since the geostationary orbit may cut through different plasma regions, such as the plasma sheet, trough, and plasmasphere. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasmasphere; polar cap phenomena

Polar observations of electron density distribution in the Earth’s magnetosphere. 1. Statistical results