An oxygen-hydrogen atmospheric model and its application to the OH emission problem

Abstract

An improved time-dependent mesospheric model has been constructed in an attempt to explain more quantitatively the main features of the OH emission. Vertical transport processes are taken into account by means of a height-dependent vertical eddy diffusion coefficient. The oxygen-hydrogen photochemical scheme employed is described in detail. The photochemistry is extended to take account of the different excited vibrational levels of OH which is assumed to be produced preferentially in the upper levels by the hydrogen-ozone reaction. On this basis, important characteristics of the observed emission are correctly predicted, including the rapid intensity decrease before sunrise, the midday maximum (comparable to the nighttime value) and the evening minimum following sunset. As with other published models, the predicted OH nightglow intensity decreases steadily throughout the night in disagreement with the generally erratic intensity variations observed at mid-latitudes. It is shown that changes in dynamic conditions, simulated by temporal variations of K by a factor of 10, produce variations in emission intensity by a factor as high as 1.8 and consequently it is concluded that changes in the dynamic structure of the emission region may be responsible for the longer term nighttime deviations from the simple model. The predicted height profiles of OH† concentration for high and low vibrational levels are quite different by day, with the former extending downward as low as 50–60 km, while the latter cut-off sharply below 80 km as do the nighttime profiles for all vibrational levels. Since these daytime distributions are sensitive to the modes of vibrational quenching, it is suggested that observations of the profiles would be of value in deciding between alternative quenching schemes.