Variability of dayside electron temperature at Venus

Langmuir probe measurements on Pioneer Venus Orbiter show that electron temperature (Te) profiles exhibit two distinct regions. The lower, but more extended region is in the main ionosphere where Te increases slowly with altitude. The other, less extended region is in the ionopause, where Te rise sharply with altitude. If horizontal magnetic fields and flux ropes in the ionosphere inhibit vertical thermal conductivity sufficiently, then the observed Te profile could be explained with EUV as the major heat source (Cravens et al., 1980). The rise in Te in the ionopause region has generally been attributed to solar wind heating (Brace and Kliore, 1991). We suggest that this sharp rise in Te is due primarily to the steep fall in electron density, Ne. If the heating rate is essentially unchanged and heat conduction is not of primary importance, then a steep rise in Te will maintain a constant electron cooling rate for a steeply falling Ne. We have observed large orbit to orbit variations in Te in the ionopause region which are found to be inversely related to changes in Ne. Variations in solar wind dynamic pressure do not seem to have a direct effect on Te, rather the effect is indirect coming through the sharp decrease in Ne.