Subauroral electron temperature enhancement in the nighttime ionosphere
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 ( ag-24-1871-img1.gif0)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 ag-24-1871-img4.gif0K; width at half this peak value ag-24-1871-img5.gif0 deg; distance between equatorward boundary and maximum ag-24-1871-img6.gif0 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.