Venus thermospheric response to short-term solar variations

Abstract

From a study of Pioneer Venus Orbiter Atmospheric Drag (OAD) data, the weak response of the atmosphere to short-term variations associated with the 27-day rotation of the sun has been clearly detected. Thermospheric temperature residuals relative to the mean diurnal variation have been found to be associated with variations in a commonly used solar activity index 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 the order of 25K. The amplitude of 11-year variations is very uncertain due to the absence of in situ data near solar minimum. These results are compared with NCAR theoretical models of the Venus thermosphere that include non-LTE processes. It is found that eddy cooling would give too strong of a response to solar variations and therefore is ruled out as the major cooling mechanism. The small amplitude of the 27-day oscillations combined with the cooling necessary for observed 300K dayside temperatures may only be explained by very strong cooling from collisional excitation of the CO 2 bending mode by atomic oxygen with subsequent radiation at 15 microns. This mechanism is even more effective since EUV increases lead to higher O/CO 2 ratios. The individual effects of this cooling mechanism and eddy cooling on the short-term thermospheric response are investigated and compared with observations. Implications concerning the 11-year thermospheric variability are also discussed. The unexpectedly strong radiative cooling mechanism isolated here may also result in even stronger cooling of the earth's mesosphere and thermosphere as CO 2 doubles in the next century than predicted by Roble and Dickinson (1989)/27/. The Mars atmosphere should not be as strongly affected by this cooling mechanism as Venus due to lower O/CO 2 ratios on Mars.