A study of Pioneer Venus orbiter atmospheric drag (OAD) data establishes that there is a weak but clear response of the atmosphere to short-term variations associated with the 27-day rotation of the Sun. Thermospheric temperature residuals relative to the mean diurnal variation have been found to be associated with variations in solar activity after correction for the Earth-Sun-Venus angle. All of the dayside OAD thermospheric data are found to exhibit this characteristic with peak-to-peak temperature variations of approximately 25 K. These results are compared with National Center for Atmospheric Research theoretical models of the Venus thermosphere that include processes not in local thermodynamic equilibrium. It is found that the low amplitude of the 27-day oscillations, combined with the cooling necessary for observed 300-K dayside temperatures, may not be explained by eddy conduction cooling and may only be explained by very strong 15-&mgr;m cooling. This presumably results from collisional excitation of the (&ngr;2=1)CO2 bending mode by atomic oxygen with subsequent radiation at 15 &mgr;m. This mechanism is estimated to be 200--1000 times more effective than self-collisions of CO2 and is even more effective since increases in EUV lead to higher O/CO2 ratios.

Isolation of this cooling mechanism, which cools the atmosphere from approximately 700 K to 300 K, was only possible through a study of 27-day variations, since the magnitude of 11-year atmospheric variations is very uncertain because of the absence of in situ data near solar minimum. Implications concerning the decrease in amplitude of the 11-year Venus thermospheric variability due to the 15-&mgr;m thermostat effect are also examined in this paper. The unexpectedly strong radiative cooling mechanism isolated here should also result in substantial cooling of the Earth's mesosphere and thermosphere as CO2 doubles in the next century with even stronger effects than predicted by Roble and Dickinson (1989). The Mars atmosphere should not be as strongly affected by this thermostatic mechanism as Venus' because of the lower O/CO2 ratios on Mars, and thus larger 11-year variations are expected. ¿ American Geophysical Union 1992