One hundred four measurements of the peak electron density in the dayside ionosphere of Venus (solar zenith angle (SZA)<80¿), along with 11 Venera 9--10 measurements, were analyzed in order to establish the response of the dayside peak to the varying solar EUV flux over the maximum to minimum phase of solar cycle 21 (December 1979 to December 1986). The relationship of the peak electron density normalized to SZA=0¿ and the EUV flux index was found to be Nm=N150(FEUV/150)0.376, which is in agreement with theoretical predictions based on Chapman theory modified for photochemical processes. The corresponding dependence on solar zenith angle at any given value of FEUV was found to be Nm(SZA)=N0 cos0.511 (SZA). The altitude of the peak was found to be independent of either SZA or FEUV. When the CO+2 production rate PCO+2 was computed using the Venus International Reference Atmosphere (VIRA) upper atmosphere model and the EUV flux computed from Hinteregger's (1981) formulae, it was found that the theoretically expected value of 0.5 for Δ log (Nm/Te0.275)/Δ log (PCO+2) was not obtained. Instead, when the assumption was used that Te=Tn at the altitude of the peak, and the solar cycle corrections given in the VIRA model were applied, a log-log slope of 0.454¿0.010 was obtained, and a value of 0.485¿0.009 resulted when the VTS3 Venus thermosphere model (Hedin et al., 1983) was used.

This suggested that the assumption Te=Tn is not satisfactory. When the electron temperature T*e was extrapolated from the electron temperature probe model (Theis et al., 1983) was used. This suggested that the assumption Te=Tn is not satisfactory. When the electron temperature T*e was extrapolated from the electron temperature probe model (Theis et al., 1980) to h=140 km and a solar cycle variation of Te=T*e [1+K(F¿10.7-190)/190> was assumed. the theoretically expected value of 0.500¿0.010 was obtained for Δlog(Nm/T0.275e)/Δ log(PCO+2 for K=0.30 when the VIRA model was used and K=0.35 when the VTS3 model was employed. This result implies that the electron temperature at the altitude of the ionosphere main peak on Venus decreases by only about 25% from solar maximum to minimum, in contrast to a decrease of about 50--75% at altitudes of above 200 km. ¿ American Geophysical Union 1989