SOIR/VEx observations of water vapor at the terminator in the Venus mesosphere

The Solar Occultation in the InfraRed (SOIR) instrument onboard Venus Express sounded the Venus mesosphere and lower thermosphere using solar occultation geometry between April 2006 and December 2014. The observations were all taken at the terminator. This paper reports on the water vapor vertical distribution above the clouds and geo-temporal variations, observed across the whole Venus Express mission. Water vapor profiles are sampled between 80 and 125 km from both the morning and evening side of the terminator. Calculations of the water vapor volume mixing ratio agree with those from previous studies, having an average atmospheric profile that varies between 0.56 ppmv and 2.45 ppmv. No significant variations were observed in observations taken between the morning and evening terminator. Short term variations of several Earth days dominate the data set, varying by up to a factor 18 over a 24 h period, from ~10 ppmv to ~0.55 ppmv at an altitude of ~111 km. Similar to previous trace gas observations by SOIR (HCl, HF and SO2), no significant spatial or long term trends are observed.