ALBERT

Description: Airglow observations are a fundamental tool to study the mesospheric part of the atmosphere. In particular the OH* emission layer is subject of many theoretical and observational studies. The choice of different transition bands of the OH* emission can introduce systematic differences between these studies, hence a profound knowledge of these differences is required for comparison. One systematic difference is given by the vertical displacements between OH* profiles due to different transition bands. A previous study has shown that the vertical displacement is highly sensitive to quenching with atomic oxygen. In this work we follow up this idea by investigating the diurnal as well as the seasonal response of OH* to changes in concentrations of atomic and molecular oxygen, the two most effective quenching species of OH*. For this task we employ a quenching model to calculate vertical OH* concentration profiles from simulations made with the SD-WACCM4 chemistry transport model. From this approach we find that despite the strong impact of O and O2 quenching on the vertical OH* structure, a considerable variability between the vertical displacements of different OH* transition bands is also induced by the natural variability of the O3 and H profiles, which primarily participate in the formation of the mesospheric OH* layer. This in particular applies for the diurnal evolution of the vertical displacements, which cannot be explained by changes in abundances of OH* quenching species only. On the other hand, vertical displacements between OH* transition bands and the amount of effective O and O2 quenching show a coherent semi-annual oscillation at lower latitudes that is in phase with the seasonal variability of the diurnal migrating tide. In particular the role of O2 quenching shows a new aspect of the semi-annual oscillation that, to our knowledge, has not been discussed before. By comparison with limb radiance observations from the SABER/TIMED satellite, we find evidence for the same oscillation in the vertical displacement between different OH* transition bands and derived O concentrations. However, our model study also reveals that quenching is not the only driving process of this feature.