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The dynamics of the upper mesosphere of Venus (~85-115 km) have beencharacterized as a combination of a retrograde superrotating zonal wind (RSZ)with a subsolar-to-antisolar flow (SSAS). Numerous mm-wave single-dishobservations have been obtained and could directly measure mesosphericline-of-sight winds by mapping Doppler-shifts on CO rotational lines, but theirlimited spatial resolution makes their interpretation difficult. By usinginterferometric facilities, one can obtain better resolution on Doppler-shiftsmaps, allowing in particular to put firmer constraints on the respectivecontributions of the SSAS and RSZ circulations to the global mesospheric windfield. We report on interferometric observations of the CO(1-0) line obtainedwith the IRAM-Plateau de Bure interferometer in November 2007 and June 2009,that could map the upper mesosphere dynamics on the morning hemisphere with avery good spatial resolution (3.5-5.5"). All the obtained measurements show,with a remarkably good temporal stability, that the wind globally flows in the(sky) East-West direction, corresponding in the observed geometry either to anunexpected prograde zonal wind or a SSAS flow. A very localized inversion ofthe wind direction, that could correspond to a RSZ wind, is also repeatedlydetected in the night hemisphere. The presence of significant meridional windsis not evidenced. Using models with different combinations of zonal and SSASwinds, we find that the data is best reproduced by a dominant SSAS flow with amaximal velocity at the terminator of ~200 m/s, displaying large diurnal andlatitudinal asymmetries, combined with an equatorial RSZ wind of 70-100 m/s,overall indicating a wind-field structure consistent with but much more complexthan the usual representation of the mesospheric dynamics.

Wind mapping in Venus' upper mesosphere with the IRAM-Plateau de Bure interferometer