A diffusive-photochemical study of the mesosphere and lower thermosphere and the associated conservation mechanisms

Abstract

Results are presented for the photochemical composition of an oxygen-hydrogen atmosphere extending from 60 to 160 km, in which allowance was made for both molecular and eddy diffusion. Methane, CH 4, was included in the model to investigate the importance of this gas as a source of hydrogen in the upper atmosphere. The composition was calculated for photochemical equilibrium, diffusive-photochemical equilibrium and for diurnally varying diffusive-photochemical conditions. The eddy diffusion coefficient was assumed to increase with height up to 100 km, and then to remain constant with a value of 4 × 10 6 cm 2 sec −1. Numerical values are given for the mechanism by which the oxygen content of the atmosphere is conserved in the face of dissociative and diffusive losses. The downward flux of atomic oxygen from the thermosphere was found to converge below about 90 km, the recombination to molecular oxygen occurring via reaction with hydrogen compounds. A quasi-conservative mechanism was also found to exist for the hydrogen gases in the atmosphere, local production of water vapour and molecular hydrogen significantly reducing the net loss of hydrogen to space. The formation of molecular hydrogen in the mesosphere, which was partially due to photolysis of methane, produced a distinct source region which supplied both the thermosphere and stratosphere. Comparison of the calculated gas profiles with observations indicates the need for more extensive measurements of the gas distributions in the upper atmosphere in order to place better limits on photochemical calculations. Results are also given for the OH airglow emission and for the 5577 Å airglow of atomic oxygen.