Radiative cooling near the mesopause

Abstract

The heat budget of the region of the upper atmosphere near the mesopause at ∼85 km is determined by a balance between radiative, photochemical and dynamical effects leading to a very cold polar mesopause in the summer and a comparatively warm mesopause in the winter. There is a temperature minimum at the mesopause primarily because of the strong radiative cooling which occurs due to thermal emission by carbon dioxide in its v₂ vibrational band at 15 µm wavelength. Above 80 km this band is no longer in local thermodynamic equilibrium (LTE) so the amount of cooling depends critically on the rate at which CO₂ molecules are excited or relaxed by collision. Any estimate of radiative cooling for this region of the atmosphere, therefore, relies on knowledge of the collisional relaxation time, τ. The importance of the temperature dependence of τ was pointed out by Houghton¹. Previous calculations used a wide range of values because no direct measurements of τ had been made in the appropriate temperature range. We show here that the effect on cooling rate calculations of using values of τ measured at temperatures down to 175 K. We also estimate a radiative relaxation time for the atmosphere near the mesopause and are able to draw conclusions as to the natural lifetime of any temperature perturbation at this altitude.