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