To study long-term trends in the mesosphere and lower thermosphere (70–100 km), we established two summer length definitions and analyzed the variability over the years (2004–2020). After the analysis, we found significant trends in the summer beginning of one definition. Furthermore, we were able to extend one of the time series up to 31 years and obtained evidence of non-uniform trends and periodicities similar to those known for the quasi-biennial oscillation and El Niño–Southern Oscillation.

The sun’s radiation exhibits a cycle with a period of ~11 years. We need to understand how the atmospheric temperature and ozone react to this solar variation, as it can affect climate. The response depends on energy transfer, chemical reactions, and atmospheric motions. There is no consensus as to some details. For example, the responses depend on the local times of the variations. This study provides results of effects of local time on the responses, from 20 to 100 km, based on measurements.