A theoretical analysis of the energy budget in the lower thermosphere

The University of Michigan's diagnostic post-processor (UM-DP) developed for use with the National Center for Atmospheric Research's-Thermosphere-Ionosphere-General Circulation Model (NCAR-TIGCM) has been extended to include a thermal term analysis capability. The upgraded processor calculates the magnitudes of the individual terms in the thermodynamic equation solved by the TIGCM as a function of 3-D space and model time. In a first study using the new capability, the lower thermospheric heating and cooling terms have been examined for a diurnally reproducible TIGCM run for moderate geomagnetic activity, solar maximum, December solstice conditions. Thermal terms calculated for geomagnetically quiet and active TIGCM runs have also been examined to investigate the geomagnetic activity dependence of the important nitric oxide (NO) radiational cooling term. Finally, the one-dimensional global mean model of Roble and Dickinson (1989) has been used to calculate the effects on the lower thermospheric thermal balance caused by the combination of natural and anthropogenic forcings projected over the next 30 yr. The principal results of this study of lower thermospheric energetics are as follows. (1) Lower thermospheric heating and cooling terms have complex morphological dependencies on latitude, longitude, altitude, geomagnetic activity, and season. (2) For the highest altitudes considered (∼ 175 km), heating caused by minor species chemistry plays the most important role in sunlit conditions, with direct solar EUV heating and Joule heating having secondary roles. The primary cooling terms at these altitudes are adiabatic expansion, NO cooling, and downward heat conduction. (3) At ∼ 125 km altitude, direct solar insolation and Joule heating are the most important heating terms, with compressional heating also contributing significantly in the winter hemisphere. The NO and CO₂ radiational terms are roughly equal in magnitude and together dominate the cooling, with adiabatic expansion being of significance at high summer latitudes. (4) At the lowest altitudes considered (∼ 103 km), direct solar insolation, heat conduction, and adiabatic compressional effects dominate the heating. The dominant cooling term here is caused by CO₂ radiation, with heat advection and adiabatic expansion in the summer hemisphere playing minor roles. (5) The important NO cooling rates can double globally for high levels of geomagnetic activity, with values at low latitudes rising from ∼160K/day to ∼400K/day at ∼150 km altitude. (6) Solar-cycle-dependent changes in NO radiational cooling and EUV heating tend to cancel each other out near ∼ 150 km altitude. In this altitude region, long-term temperature reductions caused by anthropogenic CO₂ increases may become more readily measurable, owing to the smaller masking effects of solar activity variations.