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OH airglow is an important nocturnal emission of the Earth's mesopauseregion. As it is chemiluminescent radiation in a thin medium, the populationdistribution over the various roto-vibrational OH energy levels of theelectronic ground state is not in local thermodynamic equilibrium (LTE). Inorder to better understand these non-LTE effects, we studied hundreds of OHlines in a high-quality mean spectrum based on observations with thehigh-resolution Ultraviolet and Visual Echelle Spectrograph at Cerro Paranal inChile. Our derived populations cover vibrational levels between v = 3 and 9,rotational levels up to N = 24, and individual $\Lambda$-doublet componentswhen resolved. As the reliability of these results critically depends on theEinstein-A coefficients used, we tested six different sets and found clearsystematic errors in all of them, especially for Q-branch lines and individual$\Lambda$-doublet components. In order to minimise the deviations in thepopulations for the same upper level, we used the most promising coefficientsfrom Brooke et al. (2016) and further improved them with an empiricalcorrection approach. The resulting rotational level populations show a clearbimodality for each v, which is characterised by a probably fully thermalisedcold component and a hot population with rotational temperatures between about700 (v = 9) and 7,000 K (v = 4). The latter causes non-LTE contributions at lowN, which can be estimated quite robustly based on the two-temperature model.The bimodality is also clearly indicated by the different dependence of thepopulations on changes in the effective height of the OH emission layer.

OH level populations and accuracies of Einstein-A coefficients from hundreds of measured lines