Seasonal variability of the OH Meinel bands

Seasonal and latitudinal variations of the OH Meinel bands nightglow emission are studied with a two-dimensional dynamical and photochemical coupled numerical model. The OH Meinel bands are believed to be excited primarily by the reaction between ozone and atomic hydrogen and their variations are shown to be generally driven by the atomic oxygen mixing ratio variability in the 85-90 km region, which is in turn controlled by transport processes. At low and mid-latitudes, vertical diffusion due to small-scale gravity wave breaking dominates the transport of odd-oxygen, leading to a semi-annual variation of the atomic oxygen and hence of the OH nightglow. At high-latitude, vertical advection reaches its maximum in the 85-90 km region, imposing an annual variation on the nightglow with a minimum in summer and a maximum in winter. The possible role of the reaction of HO₂ with O as an additional production mechanism of excited OH in vibrational levels lower than 6 is also investigated. For those levels, atomic oxygen and atomic hydrogen compete in driving the seasonal variation at the Meinel bands. At low-latitude they nearly cancel each other while at mid- and high-latitude the calculated OH^* exhibits features from both H and O variations. The amplitude expected in this case is weak, in the range of 10-25%, compared to 20-40% for the levels populated by the hydrogen-ozone mechanism alone. Comparisons of the model predictions with observations provide an important test for the theoretical understanding of mesospheric transport processes, particularly the relative roles of diffusive and advective transport.