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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. 

Abstract. The Earth's mesopause region between about 75 and 105 km is characterised by chemiluminescent emission from various lines of different molecules and atoms. This emission was and is important for the study of the chemistry and dynamics in this altitude region at nighttime. However, our understanding is still very limited with respect to molecular emissions with low intensities and high line densities that are challenging to resolve. Based on 10 years of data from the astronomical X-shooter echelle spectrograph at Cerro Paranal in Chile, we have characterised in detail this nightglow (pseudo-)continuum in the wavelength range from 300 to 1800 nm. We studied the spectral features, derived continuum components with similar variability, calculated climatologies, studied the response to solar activity, and even estimated the effective emission heights. The results indicate that the nightglow continuum at Cerro Paranal essentially consists of only two components, which exhibit very different properties. The main structures of these components peak at 595 and 1510 nm. While the former was previously identified as the main peak of the FeO “orange arc” bands, the latter is a new discovery. Laboratory data and theory indicate that this feature and other structures between about 800 and at least 1800 nm are caused by emission from the low-lying A′′ and A′ states of HO2. In order to test this assumption, we performed runs with the Whole Atmosphere Community Climate Model (WACCM) with modified chemistry and found that the total intensity, layer profile, and variability indeed support this interpretation, where the excited HO2 radicals are mostly produced from the termolecular recombination of H and O2. The WACCM results for the continuum component that dominates at visual wavelengths show good agreement for FeO from the reaction of Fe and O3. However, the simulated total emission appears to be too low, which would require additional mechanisms where the variability is dominated by O3. A possible (but nevertheless insufficient) process could be the production of excited OFeOH by the reaction of FeOH and O3.