Composition of the Mesosphere and Lower Thermosphere

Abstract

It is usually assumed that the atmosphere is mixed up to the turbopause and that to that altitude there is no important change in the relative amounts of the major constituents of the atmosphere. Above the turbopause, diffusive separation exists and the composition rapidly changes with increasing altitude. In the atmosphere the change probably does not occur sharply and the transition may be spread over an altitude range of as much as 30 km. For example, hydrogen, helium and argon probably start to separate at a lower altitude than the constituents whose molecular weights are closer to the atmospheric mean molecular weight. Thus, it has been suggested that the term mixtopause be used for the altitude of separation, which may be different from the turbopause. Although most models of the thermosphere have assumed a single atmospheric bulge, either near the the equator or following the sun (a summer bulge), analysis of some high altitude satellite data has suggested that there may also be a winter bulge.

A major interest in the composition in the mesosphere and lower thermosphere centers on the minor constituents. These are very important with regard to physical processes such as UV absorption, ionization, sporadic-E and so forth. Although at the present time there are big gaps in our knowledge of the minor constituents, we can make useful estimates of the species and related number densities. In the mesosphere the ozone content is of major importance. The atomic oxygen density increases rapidly with altitude above 80 km and above 120 km it is a major constituent. The rare gases are minor constituents throughout this region. It seems probable that there are layers of metallic atoms (including Na, Ca, Mg, Si, Fe) at altitudes near 100 km. Up to about 80 km water molecules and complexes containing water molecules probably exist. Other minor constituents which probably only exist in significant quantity up to 90 km include CO, CO₂, N₂O, NO, NO₂, SO₂, NH₃, CH₄ and related molecules.