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Ann. Geophys., 20, 1711-1724, 2002
www.ann-geophys.net/20/1711/2002/
© European Geosciences Union 2002
Polar observations of electron density distribution in the Earth’s magnetosphere. 1. Statistical results
H. Laakso1, R. Pfaff2, and P. Janhunen3
1ESA Space Science Department, Noordwijk, Netherlands
2NASA Goddard Space Flight Center, Code 696, Greenbelt, MD, USA
3Finnish Meteorological Institute, Geophysics Research, Helsinki, Sweden
Correspondence to: H. Laakso (hlaakso@so.estec.esa.nl)
Abstract. 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)
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