Photochemical heating of the mesosphere and lower thermosphere

A diurnally varying photochemical-diffusive model of a neutral oxygen-hydrogen mesosphere and lower thermosphere has been used to calculate the heating rates resulting from the absorption of solar ultraviolet radiation. The model extended from 60 to 160 km and permitted a coherent study to be made of this region, unlike previous investigations. In the thermosphere, where most of the absorption is by O₂, only about 35 % of the solar energy input appeared as local heating. The remainder of the input was primarily removed as chemical energy in the form of a downward flux of atomic oxygen into the mesosphere. This chemical energy was converted into heating by reactions occurring near the mesopause. In the mesosphere, where absorption by O₃ becomes important, a significant amount of the heating is realized indirectly by chemical reactions. An important consequence of this indirect heating is that it continues at night between about 80 and 100 km, unlike the situation in the thermosphere where there is almost no nocturnal heating in the model. Airglow emissions, primarily by electronically excited O₂ in the thermosphere and especially by vibrationally excited OH in the mesosphere, represent important cooling mechanisms in the model. In particular it is estimated that at 88 km. the OH airglow emission could remove about 30 % of the local energy input averaged over 24 hours.