Séminaire

Alex Panka, spécialiste des émissions non-LTE de la haute atmosphère de la Terre vient travailler à Paris sur la haute atmosphère des planètes géantes. Il a travaillé aux Etats-Unis dans l'équipe dont est issu Artem Feofilov.

In this seminar, I will present the formation mechanisms of infrared (IR) radiation in the middle and upper atmospheres (MUA) of planets. In the lower atmosphere, the inelastic molecular collisions determine the population of molecular levels. As a result, local thermodynamic equilibrium (LTE) exists where the vibrational level populations follow the Boltzmann law at the local kinetic  temperature. In the MUA, the frequency of collisions is lower, and the populations must be calculated taking into account all the processes, which populate and depopulate vibrational levels: absorption and emission of radiation, (photo)chemistry, and collisional energy exchanges.

The altitude, above which the LTE approximation is not applicable for the vibrational level populations in planetary atmospheres, corresponds approximately to 0.1–1.0 Pa pressure level (~80–90 km for Earth). Detailed treatment of the non-LTE plays a crucial role in estimating the
energetic effects of the IR radiation as well as for the diagnostics of the space-based IR observations. For the limb and even nadir satellite observations, the contribution of the non-LTE atmospheric layers to the registered radiance affects the interpretation of the lower atmosphere
measurements.

The introductory part of the seminar will be followed by presentation of recent developments in this field, and the emphasis will be given to the CO2 and OH non-LTE models in the nighttime MUA of Earth, specifically to a new energy transfer mechanism which couples OH and CO2 IR emissions. I will present the implications this mechanism has on the retrieval of OH and O densities. This will be followed by a brief discussion of CO2 and CH4 non-LTE models of Mars and Titan, respectively, as well as an overview of CH4 non-LTE emissions on Jupiter and Saturn.

marie-pierre.lefebvre@lmd.jussieu.fr