Vertical winds in the thermosphere within the polar cap

Abstract

Vertical winds measured in the upper and lower thermosphere above the South Pole station show a predominantly diurnal variation with an average amplitude of 40 m/s and 10 m/s, respectively. Downward motion was typical of the dayside polar cap in the vicinity of the cusp and cleft, and upward motion of about the same magnitude occurred in the midnight sector. Observations during the June 1991 storm period showed that the amplitude of the diurnal variation was well correlated with the daily sum of K p or ΣK p, and also that the downward wind was the most sensitive to K p change. Vertical winds in excess of 150 m/s were observed on the most active day. These measurements bear strong similarities to vertical wind data from Longyearbyen, Svalbard, at a similar geomagnetic latitude in the northern hemisphere. It was found that the downward vertical wind was proportional to the calculated divergence of the horizontal wind with a constant of proportionality equal to about twice the typical scale height at the altitude of measurement. Following the arguments of Burnside et al. (1981) and Rees et al. (1984b), we show that there is good evidence that the observed vertical winds are driven by divergence in the horizontal wind.