On the spatial distribution of some hydrogen components in the mesosphere and lower thermosphere

On the basis of a photochemical oxygen-hydrogen atmosphere model the concentrations of OH, HO2, H2, H2O and H were computed as a function of height, latitude and season. The computations were made for two values of the hydrogen/air mixing ratio, differing by a factor of about 100. Atomic hydrogen was found to be the major hydrogen constituent above about 85 km, while water vapor takes up almost all hydrogen at levels below about 75 km. In the shallow layer between these two regimes molecular hydrogen enters as the major hydrogen component. The seasonal and latitudinal variation was found to be relatively small. The effect of the air motion is discussed. An ascent of 1 cm/s near the high latitude summer mesopause seems to be necessary to keep the moisture content high enough for ice clouds to form.