New perspectives in mesospheric wave dynamics and oxygen photochemistry

Licentiate thesis, 2020

The mesosphere is the region of the atmosphere between 50km to 100km, where both dynamical and photochemical aspects play important roles for the thermal balance. This thesis focuses on the following three areas for mesospheric studies: wave dynamics, oxygen photochemistry and retrieval using the optimal estimation method.

Atmospheric gravity waves are internal disturbances in the medium that propagate horizontally and vertically. Based on linear wave theory, this thesis attempts to enhance our understanding of the relationships between the wave characteristics, the mean flow and the sources. We try to emphasise the frequency change due to the Doppler effect in several reference frames. This thesis proposes a consistent framework for deriving those wave parameters that cannot be obtained from a single type of instrument due to their particular observational geometry. Finally, a plausible interpretation of a readily available ground-based lidar observation is given as an example.

Oxygen photochemistry is another important aspect in this thesis. The underlying chemical reactions are affected by disturbances in the local temperature and density, which in turn changes the distribution of the excited oxygen species. In this work, a photochemical model has been implemented, which describes most of the important processes such as O3 photolysis that are related to the production and loss of O(1D), O2(b1Σg+ ) and O2(a1∆g).

The observation of airglow emissions provides an opportunity to explore the chemical composition and wave dynamics in the upper mesosphere. The Odin satellite has been routinely measuring O2(a1∆g) airglow emissions since 2001. In this thesis, data collected by OSIRIS are explored. Inversions are carried out in order to retrieve the volume emission rate of O2(a1∆g) as well as the mesospheric ozone density. The resulting ozone profiles are shown to be consistent with other independent ozone datasets collected by instruments aboard the same spacecraft as well as ACE-FTS and MIPAS, despite intrinsically different measurement principles. The overall good agreement between them illustrates the good performance of the retrieval technique. Furthermore, these investigations serve well as a preparatory activity for the upcoming satellite mission MATS, set for launch later this year.