Winds in the thermosphere of the Northern Polar Cap

Abstract

A preliminary report is presented here of a sequence of measurements of the thermospheric wind in the Northern Polar Cap covering nine complete periods of 24 h. This unique data set covers all local times and a range of magnetic activities with K_p values from 0 to 6⁻. Certain major features of the wind pattern were found to be independent of magnetic activity: night-time winds were steady and approximately antisolar, whereas in the morning and evening sectors the wind weakened and the direction became variable as if the observation point passed through a vortex. On the dayside, the wind was northwestward in quiet conditions but tended towards the antisolar direction with increasing activity. The simplicity of the wind pattern in geomagnetic coordinates and its similarity to the published maps of ion drift strongly suggest that the momentum of the neutral gas is supplied from active ion drag and hence the energy of circulation comes from the magnetospheric electric field.