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Case studies of the equatorward movement of the main F region trough are presented, using data recorded by a digital ionospheric sounder located at Halley Bay (76°S, 27°W; Λ = 61.8, L = 4.2). Three events were chosen to give a range of magnetic conditions and local times. Comparison of the results, projected into the equatorial plane, with theoretical predictions of the simple ‘teardrop’ model of the plasmapause, shows remarkably good agreement for the two occasions when the level of magnetic activity remained approximately constant over the observing period. Consequently, estimates of the strength of the dawn-dusk convection electric field in the magnetosphere have been made. However, magnetic activity changed during the course of the other event and the comparison gave poor results. The main F region trough results are also compared with the predictions of contemporary empirical equations which express the invariant latitude of the trough minimum as a function of local time and magnetic index, Kp, based on global satellite surveys. It is shown that none of these equations adequately accounts for the observed equatorward motion of the trough for these three cases. In particular, they contain insufficient allowance for the observed strong dependence of equatorward velocity upon magnetic activity. Some suggestions upon possible improvements to the empirical models are made which, if implemented, would be of great value for improving the reliability of predictions for high frequency, transauroral radio wave communications.