Challenging physics at the boundary between the Earth's atmosphere and Space

The extended mesosphere and lower thermosphere (MLT-X) region, i.e., between 60 and 200 km altitude, is the region where the Earth's atmosphere meets Space. This region is too low/high for in situ measurements with satellites/balloons. In the last few decades, MLT-X altitudes have been explored with remote sensing techniques by a relatively low number of ground-based instruments (radars, lidars, imagers), by a handful of satellites, as well as by occasional sounding rockets. Though, the understanding of the MLT-X is crucial for understanding the link between lower atmosphere's weather and climate and the upper atmosphere and space. Scientific questions in this region have to be jointly addressed by hydro-, thermo-, and electro-dynamic principles and other physical peculiarities, thus by the combination of different physical disciplines. The most spectacular feature resulting from the ensemble of these processes is the polar summer mesopause region where temperatures are as low as 120 K and deviate from radiative equilibrium by more than 100 degrees.

We will introduce the main "known" characteristics of the MLT-X and stress the importance of this region to global climate change studies, space weather, and to sub-orbital or very-low orbital space environment applications (e.g., commercial sub-orbital flights, re-entry of satellites). Then we will focus on two specific observational events that are challenging the current theoretical understanding of processes in this region. The first topic deals with the observations of extreme vertical drafts in the mesosphere (~80-90 km altitude), showing values more than five times larger than their standard deviation (e.g., Chau et al., 2021). The second topic is related to enigmatic plasma waves in the low-latitude lower thermosphere (around 150 km altitude) (Chau and Kudeki, 2013). While the former points to the non-linear interplay of gravity waves and turbulence motions, the latter is believed to be a result of the interplay between plasma physics, photochemistry, composition, and lower atmospheric dynamics.