Thermospheric wind measurements in the polar region

Abstract

Ion and neutral winds at an altitude of about 225 km in the polar cap F-region have been determined from the motion of barium and strontium tracers released from sounding rockets launched in 1971 and 1972. The neutral wind measured at 0800 local time, 67° north geographic latitude, and just equatorward of the polar cusp, was 94 m/s directed about 51° west of geographic north. Motion of a second cloud deployed farther north during the same flight indicated that the velocity of an air cell which has passed through the polar cusp was affected very little by the local perturbation. An analysis of the motion of the equatorward cloud using a local approximation and including ion drag and Coriolis terms is reasonably consistent with models based on satellite drag and recent in situ measurements of the thermospheric temperature distribution. Due to the low ion density in the winter ionosphere the wind was geostrophic and had a large poleward component even at 0800 where the pressure gradient was southwest. These data, taken together with the ion drag due to enhanced density from particle precipitation in the cusp and anti-sunward ion flow across the polar cap, suggest that the thermospheric winds blow strongly from day to night across the winter polar cap. Winds measured near midnight during two events in August were directed south geographically and had magnitudes of 254 m/s and 333 m/s. These velocities are more than a factor of two higher than predicted from thermospheric pressure gradients driven solely by solar thermal effects and could be due to distortion of the thermospheric temperature distribution by heating in the auroral oval. Another possibility is that high ion velocities in the polar cap, coupled with enhanced plasma density due to particle precipitation in the polar cap, put the neutral air in motion at higher latitudes. The inertia in this motion could then keep the flow anti-sunward even when the ion density has decayed sufficiently that ions no longer can strongly effect the neutrals. The analysis suggests that inertial terms as well as ion drag and non-solar heating are important in any attempt to construct a detailed model of high latitude thermospheric winds.