Local thermodynamic equilibrium of carbon dioxide in the upper atmosphere

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Abstract
The rotational and vibrational temperatures of the v2 and 2v2 modes of carbon dioxide in the upper atmosphere have been derived from transmission spectra measured by the ATMOS instrument on Spacelab 3 over the height range 60–110 km. The rotational and vibrational temperature profiles are found to be nearly identical, which leads to the conclusion that the CO2 (v2) level is in local thermodynamic equilibrium (LTE) within the experimental errors up to 95 km, and very nearly so to at least 110 km. The CO2 (2v2) level is found to be close to LTE up to at least the upper height limit of its retrieval, i.e., 93 km. The interpretation of those measurements using a non‐LTE model [López‐Puertas et al., 1986] supports a fast rate for the deactivation of CO2(v2) by atomic oxygen, similar to that derived by Sharma and Wintersteiner [1990]. This provides independent confirmation, from a qualitatively different type of measurement, of the conclusion of those authors that the radiative cooling of the upper atmospheres of the Earth is around an order of magnitude greater than our 1986 model predicted. It also has implications for thermospheric cooling on the other terrestrial planets, as well as expanding the possibilities for future remote sensing of the temperature of the upper mesosphere and lower thermosphere, since the upper limit which can be sounded is higher than previously believed.