Neutral Composition in the Upper Atmosphere

The thermosphere differs from the lower atmosphere insofar as it is heterogeneous. Below about 85 km, the atmosphere is well mixed by turbulence. Up to this height, the gases occur in roughly the same proportions that they do at ground level. Above this altitude, turbulence is no longer able to mix the atmosphere, and the density of individual species decreases at a rate that is usually fairly close to the scale height of that particular species. The thermosphere is, however, very dynamic. Heating, as a result of the absorption of solar EUV radiation and Joule heating, causes large horizontal winds. Additional wind forcing comes from the coupling between ions and neutrals at high latitudes. These winds diverge and converge as they flow between hot and cold regions, which sets up a vertical wind system that is the primary mechanism for driving composition change as these vertical winds act on the height gradients of composition. This chapter describes these processes in detail, describes some of the ways this forcing changes, and then discusses hydrogen, a gas that is affected in a different way by these processes.