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. Spectroscopy of the hydroxyl (OH) airglow has been acommonly used way to remotely sense temperatures in the mesopause region formany decades. This technique relies on the OH rotational state populationsto be thermalized through collisions with the surrounding gas into aBoltzmann distribution characterized by the local temperature. However,deviations of the rotational populations from a Boltzmann distributioncharacterized by a single temperature have been observed and attributed toan incomplete thermalization of the OH from its initial, non-thermodynamic-equilibrium distribution. Here we address an additional cause for theapparent amount of excess population in the higher rotational levels of theOH airglow brought about by integrating these OH emissions through verticalgradients in the atmospheric temperature. We find that up to 40 % of theapparent excess population, currently attributed to incompletethermalization, can be due to the vertical temperature gradients created bywaves. Additionally, we find that the populations of the different uppervibrational levels are affected differently. These effects need to be takeninto account in order to assess the true extent of non-thermodynamic-equilibrium effects on the OH rotational populations.

Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements