Resolving the mesospheric nighttime 4.3 µm emission puzzle: comparison of the CO₂(ν₃) and OH(ν) emission models

Abstract. In the 1970s, the mechanism of vibrational energy transfer from chemically produced OH(ν) in the nighttime mesosphere to the CO₂(ν₃) vibration, OH(ν) ⇒ N₂(ν) ⇒ CO₂(ν₃), was proposed. In later studies it was shown that this "direct" mechanism for simulated nighttime 4.3 µm emissions of the mesosphere is not sufficient to explain space observations. In order to better simulate these observations, an additional enhancement is needed that would be equivalent to the production of 2.8–3 N₂(1) molecules instead of one N₂(1) molecule in each quenching reaction of OH(ν) + N₂(0). Recently a new "indirect" channel of the OH(ν) energy transfer to N₂(ν) vibrations, OH(ν) ⇒ O(¹D) ⇒ N₂(ν), was suggested and then confirmed in a laboratory experiment, where its rate for OH(ν = 9) + O(³P) was measured. We studied in detail the impact of the "direct" and "indirect" mechanisms on CO₂(ν₃) and OH(ν) vibrational level populations and emissions. We also compared our calculations with (a) the SABER/TIMED nighttime 4.3 µm CO₂ and OH 1.6 and 2.0 µm limb radiances of the mesosphere–lower thermosphere (MLT) and (b) with ground- and space-based observations of OH(ν) densities in the nighttime mesosphere. We found that the new "indirect" channel provides a strong enhancement of the 4.3 µm CO₂ emission, which is comparable to that obtained with the "direct" mechanism alone but assuming an efficiency that is 3 times higher. The model based on the "indirect" channel also produces OH(ν) density distributions which are in good agreement with both SABER limb OH emission observations and ground and space measurements. This is, however, not true for the model which relies on the "direct" mechanism alone. This discrepancy is caused by the lack of an efficient redistribution of the OH(ν) energy from higher vibrational levels emitting at 2.0 µm to lower levels emitting at 1.6 µm. In contrast, the new  indirect  mechanism efficiently removes at least five quanta in each OH(ν ≥ 5) + O(³P) collision and provides the OH(ν) distributions which agree with both SABER limb OH emission observations and ground- and space-based OH(ν) density measurements. This analysis suggests that the important mechanism of the OH(ν) vibrational energy relaxation in the nighttime MLT, which was missing in the emission models of this atmospheric layer, has been finally identified.