Photochemical modeling of nonmigrating tides in the 15 μm infrared cooling of the lower thermosphere over one solar cycle and comparison with SABER

Abstract:

Tidal diagnostics of SABER CO₂ 15 μm data shows a substantial modulation of the energy budget of the lower thermosphere due to nonmigrating tides: relative amplitudes of the CO₂ cooling rates for the DE2 and DE3 components are on the order of 15-50% with respect to the monthly mean emissions. Supporting photochemical tidal modeling using TIME-GCM and the empirical CTMT model reproduces the general amplitude structures and phases. Furthermore, it indicates that the main tidal coupling mechanism is the temperature dependence of the collisional excitation of the CO₂ (01101) fundamental band transition (ν₂). The response to neutral density variations is as important as temperature above 115 km as such explaining an unexpected tidal phase behavior in the observation. The contribution of vertical advection is comparatively small. In order to test the sensitivity of the modeled DE2 and DE3 CO₂ VER tides to the solar cycle and to the specific choice of mean temperature, atomic oxygen, and CO₂ density, we extend the modeling by using background from MSIS, SABER, and SCIAMACHY. The results indicate that the current uncertainties in the background temperature and atomic oxygen used for the photochemical modeling do not impact our conclusion about the relative importance of the tidal coupling mechanisms. Our results quantify the response of the CO₂ 15 μm infrared cooling of the lower thermosphere to tropospheric tides and delineate the coupling mechanisms that lead to the observed strong longitudinal and local time variability.