The Venus OH Nightglow Distribution

The first identification of the OH airglow in the terrestrial mesosphere was made in 1950 by Meinel [1950]. Recently, the unexpected presence of the OH nightglow was observed in the Venus mesosphere by Piccioni et al. [2008] using a limb profile from the Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft. They clearly identified the (1-0) and (2-1) transitions at 2.80 and 2.94 µm, respectively and the (2-0) band at 1.43 µm. Additional bands belonging to the Δv=1 sequence also appear to be present longward of the (1-0) band. In a preliminary study of characteristics of the OH emission distribution, Gérard et al. [2010] pointed out a correlation between the OH(Δv=1) and the O2(a1Δ) nightglow intensities. In Soret et al. [2010], the full dataset of VIRTIS-M limb observations of the OH Venus nightglow has been corrected from the thermal emission of the planet and analyzed to determine its characteristics. Based on 3328 limb profiles, the study shows that the emission is highly variable. No clear dependence of the airglow layer altitude versus the antisolar angle is established. The peak brightness appears to decrease away from the antisolar point even if the variability at a given location is very strong. Some correlation between simultaneous observations of the intensity of the OH and the O2(a1∆) emissions has also been detected, presumably because atomic oxygen is a common precursor to the formation of O2(a1∆) and O3, whose reaction with H produces excited OH. A relation given in the one-dimensional photochemical model of Krasnopolsky [2009] has been used to link the OH and the O2(a1∆) airglows through the hydrogen flux at 130 km. It appeared that using a constant flux did not fill well the simultaneous OH and O2 observations. Either the flux has to vary with the distance to the antisolar point or other dimensions have to be involved.