Abstract. Since 2002, SABER (Sounding of the Atmosphere using Broadband Emission Radiometry)/TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics) has been continuously measuring the day- and nighttime infrared limb radiances of the mesosphere and lower thermoshere (MLT) in ten broadband channels. Recently, the MLT daytime temperature/ pressure and CO2 densities have been obtained self-consistently from SABER 15 μm and 4.3 μm emission observations. However, similar nighttime data remain unprocessed due to a lack of understanding of the 4.3 μm emission generating mechanisms. A previous study suggested the “direct” transfer OH(v)) ⇒ N2(v)) ⇒ CO2(v3)) ⇒ 4.3 μm of vibrational excitation from OH(v) to CO2 in the nighttime mesosphere. However, accounting for this excitation mechanism alone leads to significant under-prediction (by up to 80 %) of observed 4.3 μm limb radiances. Recently, theoretical and laboratory studies have suggested an additional “indirect” nighttime channel OH(v)) ⇒ O(1D)) ⇒ N2(v)) ⇒ CO2(v3)) ⇒ 4.3 μm of this energy transfer. We implemented this new channel in our non-LTE (non-Local Thermodynamic Equilibrium) model and show that, for various latitudinal and seasonal scenarios, including this additional channel brings differences between simulated and measured nighttime SABER 4.3 μm limb radiances to (−20, +30) %. These results confirm the important role of the new mechanism as a source of the nighttime 4.3 μm emission. This finding creates new opportunities for the application of CO2 4.3 μm observations in the study of the energetics and dynamics of the nighttime MLT.
Errors in Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) kinetic temperature caused by non-local-thermodynamic-equilibrium model parameters
