ZONALLY AVERAGED CIRCULATION, TEMPERATURE, AND COMPOSITIONAL STRUCTURE OF THE LOWER THERMOSPHERE AND VARIATIONS WITH GEOMAGNETIC ACTIVITY.

A zonally averaged chemical-dynamical model of the thermosphere is used to examine the effect of high-latitude particle and Joule heating on the neutral composition, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating alone is a summer-to-winter flow, with a winter enhancement in atomic oxygen. The high-latitude heat sources drive mean circulation cells that reinforce the solar-driven circulation in the summer hemisphere and oppose this circulation in the winter hemisphere. The changes in wind and temperature caused by the high-latitude heat sources increase the relative concentration of N//2 and O//2 in the high-latitude upper thermosphere and decrease the O concentration in the high-latitude lower thermosphere. For prolonged moderate levels of geomagnetic activity, the peak atomic oxygen density in the polar regions can decrease by factors of 2-3 from geomagnetic quiet conditions.