Nonmigrating tidal impact on the CO₂ 15 μm infrared cooling of the lower thermosphere during solar minimum conditions

Carbon dioxide (CO₂) infrared emissions at 15 μm is the primary radiative cooling mechanism of the thermosphere in the altitude range of 100–135 km. This paper explores the role of two important diurnal nonmigrating tides, the DE2 and DE3, in the modulation of CO₂ 15 μm emissions during the solar minimum year 2008 by (i) analyzing Sounding the Atmosphere using Broadband Emission Radiometry (SABER) CO₂ cooling rate data and (ii) photochemical modeling using dynamical tides from the empirical Climatological Tidal Model of the Thermosphere model. Tidal diagnostics of SABER data shows that the CO₂ cooling rate amplitudes for the DE2 and DE3 components are on the order of approximately 20–50% relative to the monthly means, and they maximize around the lower bound (100 km) of the analyzed height interval. The photochemical modeling reproduces the observed results, albeit with systematic amplitude differences which is likely related to the uncertainty in the model input backgrounds, especially atomic oxygen. The main tidal coupling mechanism is found to be the temperature dependence of the collisional excitation of the CO₂ ν₂ vibrational state. However, neutral density becomes equally important above ∼110 km, thereby explaining observed evanescent DE2 and DE3 phases which are not present in temperature tides. The contribution of vertical tidal advection is comparatively small. The relative importance of the coupling mechanisms is the same at all latitudes/seasons. These results indicate that upward propagating nonmigrating tides forced by latent heat release in the lower atmosphere impact the thermospheric energy budget by modulating the longitudinal/local time behavior of the CO₂ infrared cooling.