THE OH NIGHTGLOW EMISSION

Abstract

The several determinations of the relative vibrational transition probabilities for the OH vibration-rotation bands are examined. The discrepancies in the results may be attributed to their strong dependence on the observed intensities. More recent intensity measurements of the airglow, which are better suited for such a determination, indicate that terms of order higher than the first, in the usual dipole moment expansion, can he neglected. To represent the laboratory measurements, however, the higher order terms are necessary. The principal characteristics of the hydroxyl nightglow emission are reviewed. The emission rate shows a summer minimum and a winter maximum. Most of the spectroscopically determined gas temperatures exhibit a similar trend except that the amplitude of their seasonal variations shows a more distinct increase with the latitude. A comparison of the spectroscopic temperatures with the temperature profiles and emission altitudes obtained with rockets indicates that either the altitude of the hydroxyl emission layer or the temperature profile of the homosphere varies with longitude. In the middle latitudes no correlation is observed between the emission rate and the temperature whereas at 62N the temperatures greater than about 250K increase with increasing emission rate. Some authors attribute this feature to two excitation mechanisms: a temperature-insensitive reaction of atomic hydrogen with vibrationally excited molecular oxygen for middle latitudes while a temperature-sensitive reaction of atomic hydrogen with ozone would reproduce the correlation observed at high latitudes. In an attempt to test the hydrogen-ozone reaction, an examination was made of a model hydrogen-oxygen atmosphere in local photochemical equilibrium. This model leads to the conclusion that the nighttime ozone concentration, in the 65- to 95-km region, is determined by atomic hydrogen. Hydroxyl emission profiles calculated on the basis of the hydrogen-ozone reaction show that the observed emission rate can be reproduced. As the ozone concentration is determined by the hydrogen-ozone reaction the model emission rate is independent of temperature. The emission peak would, however, occur considerably below the altitudes deduced from rocket observations. If deactivation is assumed to suppress the emission below 80 km, to bring the model and rocket attitude into agreement, the model falls far short of producing the observed emission rate. In addition, the observed winter maximum in the arctic is not explained. As an alternative to the hypothesis of local photochemical equilibrium the possibility of a downward transport of aton3ic oxygen from about 110 km to the emission layer was examined. Except during the arctic winter, it appears possible that this mechanism, through the hydrogen-ozone reaction, could maintain the observed emission rate. A global circulation system would be required to bring atomic oxygen from lower latitudes into the arctic region in order to account for the emission in the arctic. The temperature-intensity relation to be expected under these circumstances is uncertain.