Electron temperature enhancement beneath the magnetospheric cusp

摘要

In the dayside polar region, soft electron precipitation and heat conduction cause a significant increase in the electron temperature of the upper ionosphere. Using DE-2 satellite data, the properties of this remarkable cusp signature are investigated. First, the location of the temperature enhancement is determined. It is found that this location is primarily dependent on the substorm activity. For geomagnetically quiet conditions (AE 0) the temperature peak is located near 79° invariant latitude. For each increase in the AE index by 100 nT it moves equatorward by about 1 degree. During strongly disturbed conditions it may be observed at a latitude as low as 61 degrees. Since the substorm activity is controlled by the Bz component of the interplanetary magnetic field, the location of the temperature peak is also dependent on this parameter. Besides the location, the magnitude of the temperature increase is of interest. This magnitude is found to be strongly dependent on altitude. Within the height range 280 to 940 km it increases by a factor of almost 5. Compared with altitude, other parameters like the dynamic pressure of the solar wind or the geomagnetic activity are only of secondary importance. To obtain mean latitudinal profiles of the cusp-related temperature enhancement, a superposed epoch type of averaging procedure is used. This way the basic latitudinal structure of this narrow feature is preserved. For an altitude of 700 km the following mean properties are derived: amplitude 1200 K; width at half this maximum value 2.5 degrees; distance between equatorward boundary and maximum of the temperature increase 3.5 degrees. In general, a decrease in the electron density is observed at the location of the temperature peak. Only in the uppermost ionosphere and during geomagnetically quiet conditions is the density observed to increase, on average. The results obtained in this study should prove very useful for both empirical and theoretical modeling of the dayside polar ionosphere. They may also shed some light on the reconfiguration of the magnetosphere in the cusp region during substorm activity.