Thermal cell structures in the high-latitude thermosphere induced by ion drag

Abstract

The occurrence of high-latitude thermospheric density structures during disturbed conditions that are more complex than the forcing itself suggests that the structure may be caused by a qualitative change in the balance of forces. Using a general circulation model of the thermosphere, we have examined the terms involved in the force balance for levels in the upper and lower thermosphere during active and quiet times and examined the thermal structure in relation to the terms that dominate the balance. A simulation reveals that where ion drag is unable to accelerate the atmosphere into rapid motion the Coriolis force dominates and for fixed pressure levels the centers of cyclonic motion are colder and denser than the surrounding air, while centers of anticyclonic motion are warmer and less dense. At fixed heights, densities are high in the evening anticyclonic gyre and low in the dawn cyclonic gyre. Another simulation reveals that this situation is radically changed when the atmosphere is spun up to rapid motion and the centrifugal force is the dominant inertial force, whence both anticyclones and cyclones become centers of relatively cold high-density air at fixed pressure levels. On constant height surfaces, cold low-density centers are found on both the dawnside and duskside with a trough of low-density air over the pole connecting them. This intrusion of low-density splits the evening high-density region that exists under quiet conditions giving the four-cell pattern found by Crowley et al. (1989a, 1996a, 1996b).