Remote Sensing in the 15 µm CO2 Band: Key Concepts and Implications for the Heat Balance of Mesosphere and Thermosphere

We investigated the algorithms and physical models currently applied to remote sensing of the mesosphere and lower thermosphere (MLT) using space-based observations of the CO2 15 µm emission. We show that the measured 15 µm radiation constrains the population of excited CO2 vibrational levels and the 15 µm radiative flux divergence in the MLT, but not the 15 µm cooling. Moreover, the models of the non-local thermodynamic (non-LTE) excitation of CO2 in the MLT contradict the laboratory studies of this excitation. We present a new model of the non-LTE in CO2 that is both consistent with the observed CO2 15 µm radiation and provides the CO2 cooling of the MLT, which aligns with the laboratory-measured rate coefficient kO of the CO2 vibrational excitation by collisions with O(3P) atoms. Its application shows that the current non-LTE models dramatically overestimate this cooling. Even for the low laboratory-confirmed rate coefficient of the CO2-O(3P) excitation, kO=1.5×10−12 s−1cm−3, excess cooling is equal or higher than the true cooling, reaches a value of 10 K/day, and is maximized in the mesosphere region around 100 km—a region which is very sensitive to any changes in the heat balance. For kO=3.0×10−12 s−1cm−3, which is currently used in the general circulation models of the MLT, excess cooling reaches 25–30 K/day. The results of this study contradict the widely held belief that the 15 µm CO2 emission is the primary cooling mechanism of the middle and upper atmospheres of Earth, Venus, and Mars. A significant reduction in 15 µm cooling will have a major impact on both the modeling of the current MLT and the estimation of its future changes due to increasing CO2. It also strongly influences the interpretation of MLT 15 µm emission observations and provides new insights into the role of this emission in the middle and upper atmospheres of Mars, Venus, and other extraterrestrial planets.