  The dynamics of the upper mesosphere of Venus (~85-115 km) have been
characterized as a combination of a retrograde superrotating zonal wind (RSZ)
with a subsolar-to-antisolar flow (SSAS). Numerous mm-wave single-dish
observations have been obtained and could directly measure mesospheric
line-of-sight winds by mapping Doppler-shifts on CO rotational lines, but their
limited spatial resolution makes their interpretation difficult. By using
interferometric facilities, one can obtain better resolution on Doppler-shifts
maps, allowing in particular to put 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,
that could map the upper mesosphere dynamics on the morning hemisphere with a
very 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 an
unexpected prograde zonal wind or a SSAS flow. A very localized inversion of
the wind direction, that 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 best reproduced by a dominant SSAS flow with a
maximal velocity at the terminator of ~200 m/s, displaying large diurnal and
latitudinal asymmetries, combined with an equatorial RSZ wind of 70-100 m/s,
overall indicating a wind-field structure consistent with but much more complex
than the usual representation of the mesospheric dynamics.

Earth and Planetary Astrophysics