The temperature of the Venus mesosphere from O2 (aΔg1) airglow observations

We have used near-infrared spectroscopic observations of the Venus nightside taken with the Infrared Imager and Spectrograph 2 (IRIS2) on the Anglo–Australian Telescope to derive temperature maps for the Venus mesosphere at an altitude of ∼95 km. The temperatures are derived from the distribution of rotational line intensities in the O2 (aΔg1) airglow band at 1.27 μm. To obtain reliable temperatures at the relatively low spectral resolution of IRIS2, we have developed a forward modeling approach to handle the blending of individual O2 lines and the telluric absorption in the same O2 band. The technique provides temperature retrievals with accuracy comparable to, or better than that of previous high-spectral resolution determinations. The resulting temperature maps show spatial temperature structure that varies from night to night, as does the intensity distribution. Intensity weighted mean temperatures range from about 181 to 196 K. The temperatures are typically 15–30 K higher than those expected from the Venus International Reference Atmosphere (VIRA) profile. The temperatures fall in regions of low O2 emission rate to values closer to the VIRA levels. Our temperatures are similar to, but slightly lower than those obtained from stellar occultation measurements with SPICAV on Venus Express. We suggest that we are seeing a region of locally enhanced temperature caused by compressional heating in the downwelling gas around the antisolar point.