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Ann. Geophys., 24, 1871-1885, 2006
www.ann-geophys.net/24/1871/2006/
© European Geosciences Union 2006
Subauroral electron temperature enhancement in the nighttime ionosphere
G. W. Prölss
Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
Abstract. In the nightside subauroral region, heat transfer from the ring current
causes a significant increase in the electron temperature of the upper
ionosphere. Using DE-2 satellite data, we investigate the properties of this
remarkable feature. We find that the location of the temperature enhancement
is primarily dependent on the level of geomagnetic activity. For
geomagnetically quiet conditions ( )the temperature peak is
located slightly poleward of 60° invariant latitude. For each decrease
in the Dst index by 10 nT, it moves equatorward by about one degree. To a
lesser degree, the location of the heating effect also depends on magnetic
local time, with a significant positional asymmetry about midnight. The
magnitude of the temperature enhancement varies with altitude. Within the
height range 280 to 940 km, the peak temperature increases by 73%, on
average. Thereby a conspicuous increase in the temperature gradient is
observed above about 700km altitude. The magnitude of the heating effect
also depends on the level of geomagnetic activity. For a decrease in the
Dst index by 100 nT, the peak temperature increases by 46%, on average.
This rate of increase, however, depends on season and is significantly
smaller during winter conditions. A superposed epoch type of averaging
procedure is used to obtain mean latitudinal profiles of the temperature
enhancement. For an altitude of 500 km, the following mean properties are
derived: amplitude  K; width at half this peak value
 deg; distance between equatorward boundary and maximum 
deg. On average, a decrease in the electron density is observed at the
location of the temperature enhancement, at least at 500 km altitude. At the
same time, a moderate increase in the zonal ion drift speed is recorded at
this location. During larger geomagnetic storms, the latitudinal profile of
the temperature enhancement assumes a more step-function-like shape, with a
broad increase in electron temperature poleward from the equatorial edge of
the electron temperature enhancement. Also the heating effects may extend to
very low latitudes (less than 35° invariant latitude). And residual
heating effects are observed long after the storm-substorm activity has
ceased. The results obtained in this study should prove useful for both
empirical and theoretical modeling of the nightside subauroral ionosphere.
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