The assumption of chemical equilibrium is widely used to derive information about poorly measured characteristics of the mesosphere–lower thermosphere from rocket and satellite data and to study the physicochemical processes at these altitudes. In this work, we analyze the fundamental aspects of chemical equilibria of two important trace gases and discuss their possible applications.

In this work, the recently developed analytical criterion for determining the boundary of nighttime ozone chemical equilibrium (NOCE) in the mesopause region (80–90 km) is used (i) to study the connection of this boundary with O and H spatiotemporal variability based on 3D modeling of chemical transport and (ii) to retrieve and analyze the spatiotemporal evolution of the NOCE boundary in 2002–2021 from the SABER/TIMED data set.

The solar cycle affects the H₂O concentration in the upper mesosphere mainly in two ways: directly through photolysis and, at the time and place of NLC formation, indirectly through temperature changes. The H₂O–Lyman-α response is modified by NLC formation, resulting in a positive response at the ice formation region (due to the temperature change effect on the ice formation rate) and a negative response at the sublimation zone (due to the photolysis effect).

Periodic changes in all hydrodynamic parameters are constantly observed in the atmosphere. The amplitude of these fluctuations increases with height due to a decrease in the atmospheric density. In the upper layers of the atmosphere, waves are the dominant form of motion. We use a model of the general circulation of the atmosphere to study the contribution to the formation of the dynamic and temperature regimes of the middle and upper atmosphere made by different global-scale atmospheric waves.

Numerical modelling is used to simulate atmospheric circulation and calculate residual mean meridional circulation (RMC) during sudden stratospheric warming (SSW) events. Calculating the RMC is used to take into account wave effects on the transport of atmospheric quantities and gas species in the meridional plane. The results show that RMC undergoes significant changes at different stages of SSW and contributes to SSW development.