Neutral Motions in the Polar Thermosphere for Northward Interplanetary Magnetic Field

On 24th November, 1982, The North-South (Bz) component of the Interplanetary Magnetic Field (IMF) became positive for a period of about 11 hours reaching a relatively large and steady value of---25 nT. During this rare occurrence, the Dynamics Explorer-2 (DE-2) spacecraft was in a configuration that enabled the dynamics of both ionic and neutral species of the high-latitude F-region to be measured simultaneously along the track of the polar-orbiting satellite. Results from two Northern (winter) polar passes of DE-2, extracted from a larger data set, are shown to illustrate the response of the neutral F-region to ion drag forcing arising from a configuration of ion convection characteristic of strongly northward IMF. The measured neutral winds differ appreciably from those more commonly observed for periods of southward IMFo The multicellular ion drift pattern associated with positive Bz is observed to drive a similar but less structured and weaker neutral wind configuration in the winter polar cap. Major features of the ion drift pattern are mimicked by the neutral circulation but smaller-scale and more irregular structures of ion flow are not. This is ascribed to the relatively long time constant (few hours) for momentum exchange between the ion and neutral gases. The results demonstrate that sunward flow of neutral gas can be established and maintained by ion drag in the central polar cap for positive Bz.