2012A&A...546A.102M

The dynamics of the upper mesosphere of Venus (∼85-115km) have been characterized as a combination of a retrograde superrotating zonal wind (RSZ) with a sub-solar-to-antisolar flow (SSAS). Numerous mm-wave single-dish observations have been obtained and been able to directly measure the mesospheric line-of-sight winds by mapping the Doppler shifts of CO rotational lines, but their limited spatial resolution makes their interpretation difficult. By using interferometric facilities, one can achieve a higher resolution in Doppler-shift maps, allowing us in particular to place firmer constraints on the respective contributions of the SSAS and RSZ circulations to the global mesospheric wind field. We report on interferometric observations of the CO(1-0) line obtained with the IRAM-Plateau de Bure interferometer in November 2007 and June 2009, which was able to map the upper mesosphere dynamics on the morning hemisphere with a high spatial resolution (3.5-5.5''). All the obtained measurements show, with a remarkably high temporal stability, that the wind globally flows in the (sky) east-west direction, corresponding in the observed geometry to either an unexpected prograde zonal wind or a SSAS flow. A very localized inversion of the wind direction, which could correspond to a RSZ wind, is also repeatedly detected in the night hemisphere. The presence of significant meridional winds is not evidenced. Using models with different combinations of zonal and SSAS winds, we find that the data is most accurately reproduced by a dominant SSAS flow with a maximal velocity at the terminator of ∼200m/s, displaying large diurnal and latitudinal asymmetries, combined with an equatorial RSZ wind of 70-100m/s. Overall, this is indicative of a wind-field structure consistent with but much more complex than the usual representation of the mesospheric dynamics.