Comparison of plasma flow and thermospheric circulation over northern Scandinavia using EISCAT and a Fabry-Perot interferometer

Abstract

The EISCAT incoherent scatter radar, operating in a full tristatic mode, provided data on the ionospheric plasma drift above northern Scandinavia, during the 24 h period, 11 UT 25 November to 11 UT 26 November 1982. For the hours of darkness, 14 UT until 05 UT, observations of thermospheric winds were made by means of a ground-based Fabry-Perot interferometer (FPI) operated at Kiruna Geophysical Institute (21° E, 68° N). During this period, the radar observations describe well the ebbing and flowing of regions of strong convective ion flow associated with the auroral oval. As individual geomagnetic disturbances occur, the overall ion flow pattern intensifies and moves equatorward. The zonal thermospheric wind observed by the FPI responds rapidly to surges of the local ionospheric convection, while the meridional wind response is slower and apparently to much larger-scale features of the geomagnetic input to the high latitude thermosphere. From the data base, periods of strong heating of the ionospheric ions and of the thermospheric gas can be identified, which can be compared with Joule and particle heating rates deduced from the observations of ionospheric drifts, neutral winds, electron densities and auroral emission rates. A three-dimensional, time-dependent global thermospheric model is used to distinguish local and global features of the thermospheric wind field. Meridional and zonal wind components at 312 km may be theoretically derived from the EISCAT data using an appropriate model (MSIS) for neutral temperature. The EISCAT-derived meridional wind is within about 50 m s −1 of the FPI observations throughout the period of joint observations. The EISCAT-derived zonal wind is systematically larger (by about 50%) than the FPI measurement, but the two independent measurements follow closely the same fluctuations in response to geophysical events until 03 UT, when the EISCAT solution is driven away from the FPI measurement by a sharp increase in both neutral and ion temperatures. Between 03 and 05 UT the EISCAT-derived zonal wind is 200–400 m s −1 westward. Allowance for the neutral temperature rise would reduce the EISCAT values towards the very small zonal winds shown by the FPI during this period. We describe the relatively straightforward analysis required to derive the meridional wind from the radar data and the limitations inherent in the derivation of zonal wind, using the ion energy equation, due to the lack of precise knowledge of the background neutral temperature from the EISCAT data alone. For analysis of EISCAT ion drift observations at 312 km, the ground-based FPI temperature measurements do not improve the accuracy of the analysis, since the median altitude of the FPI measurement is probably in the range 180–240 km throughout the observation period. This median altitude and the temperature gradient both fluctuate in response to local geomagnetic events, while the temperature gradient may be considerably greater than that predicted by standard atmospheric models. When the neutral temperature is well known, or when there is a large enhancement of the ion temperature, the EISCAT-derived zonal wind exceeds the FPI measurement, but the consistency with which they correlate and follow ion-drag accelerations suggests that the differences are purely due to the considerable altitude gradients which are predicted by theoretical models.