Modulation of dayside ion and neutral distributions at Venus: Evidence of direct and indirect solar energy inputs

Abstract

In-situ measurements of ion and neutral composition and temperature across the dayside of Venus during 1979–1980 exhibit long and short-term changes attributed to solar variations. Following solar maximum, dayside concentrations of CO + and the neutral gas temperature are relatively smoothly modulated with a 28-day cycle reasonably matching that of the solar F 10.7 and EUV fluxes. Measurements some 6–8 months earlier show less pronounced and more irregular modulation, and short-term day-to-day fluctuations in the ions and neutrals are relatively more conspicuous than in the later period. During the earlier period, the solar wind at Venu exhibits relatively large velocity enhancements, which appear to be consistent with differences in solar coronal behavior during the two periods. It is suggested that through the solar wind variations and associated changes in the draping of the interplanetary magnetic field about the dayside, fluctuating patterns of joule heating may occur, producing the observed short term ion and neutral variations. This indirect energy effect, if verified, presents a complication for quantitatively analyzing the modulation in neutral temperature and ion concentration produced by changes in direct EUV radiation.