Séminaire
Mesosphere and Lower Thermosphere (MLT) at approximately 60-120 km is an area sensitive both to solar influence and to radiation coming from below. It might be also called a gateway between the lower atmosphere and space.
Understanding the energy balance of the region requires estimating the radiative sources and sinks for CO2, O3, and H2O and taking into account the effects of O2/O3 photolysis. However, calculating these energy balance terms is complicated by so called “non-LTE” effects. In the lower atmosphere inelastic molecular collisions determine the
population of molecular levels. As a result local thermodynamic equilibrium (LTE) exists where the populations obey the Boltzmann law with the local kinetic temperature. In the middle and upper atmosphere the frequency of the collisions is lower and other processes which populate and depopulate vibrational levels (absorption and emission of radiation in
molecular bands, redistribution of excitation between colliding molecules, chemical excitation) must be taken into account. The LTE is no longer valid and the populations must be found by solving the system of equations expressing the balance of above-mentioned processes which populate and depopulate vibrational levels.
The current status of the non-LTE models for CO2, O3, and H2O molecules will be presented. Applications of these models to the interpretation of the SABER multichannel broadband radiometric instrument (NASA TIMED satellite) measurements in MLT will be shown. Correlations of the polar summer mesospheric temperatures with the polar mesospheric clouds (PMCs) at ~80 km altitude will be demonstrated. Possible effects of the non-LTE radiances in MLT on the interpretation of the lower atmospheric measurements will be discussed.
Thibaud Thonat - 01 69 33 51 61
Paul-Etienne Mallet - 01 69 33 51 05