Venus’ upper atmospheric dynamical structure from ground-based observations shortly before and after Venus’ inferior conjunction 2009

Investigations on the dynamical structure of Venus upper atmosphere were carried out by infrared heterodyne Doppler wind measurements shortly before and after the venusian inferior conjunction on March 27, 2009.The Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) has been installed at the McMath–Pierce Solar Telescope on Kitt Peak, Arizona, USA to detect non-local thermodynamical equilibrium (non-LTE) emission lines of CO2 at a wavelength of 10.5 μm. These solar induced emission lines originate at a pressure level of 1 μbar corresponding to an altitude level of 110 ± 10 km. From the frequency position of the spectral lines we directly derived Doppler winds without any additional information. The high spatial resolution with a field-of-view of 1.6 arcsec compared to an apparent diameter of Venus of 57 arcsec allowed to collect information at different latitudes of the illuminated planet.Line of sight wind velocities between 189 ± 11 m/s and 41 ± 14 m/s were detected along the illuminated evening (western) limb in March and along the bright morning (eastern) limb in April. Single observations at the evening and morning terminator do not show a systematic difference of wind velocities. The measured wind is uniform at low and mid latitudes. In March a lower mean value of 134 ± 1 m/s was found compared to April where we retrieved a value of 141 ± 1 m/s. Poleward of a latitude of 50° we observed a strong decrease in wind speed down to 41 ± 14 m/s.In addition to the pure line of sight wind velocities we used the observing geometry for additional interpretations regarding a global flow from the subsolar point to the antisolar point (SS–AS flow) and a global retrograde superrotational zonal wind (RSZ). The estimations indicate a dominating SS–AS flow with a maximum wind velocity at the terminator of 138 ± 1 m/s at low and mid latitudes. No indication of a global RSZ component was found. Corresponding wind values for the latter yield wind velocities in the zonal direction between+20 m/s (retrograde direction) and −20 m/s (prograde direction) at different latitudes. An inversion of the wind direction is in disagreement with a global RSZ behavior.The comprehensive dataset was used to investigate short term wind variabilities and changes up to 58 m/s within few days were found. We included a detailed comparison of concurrent single position observations with sub-millimeter measurements (Clancy, R.T., Sandor, B.J., Moriarty-Schieven, G. [2012]. Icarus 217, 794–812) suggesting a cross terminator gradient at certain latitudes. A detailed interpretation of the observed time dependent behavior by global circulation models including wave activities will be addressed in future work.

► Observed line-of-sight wind velocities are between 189 ± 11 m/s and 41 ± 14 m/s.
► Wind velocities are constant at latitudes lower than 50° (mean value: March 134 ± 1 m/s, April: 141 ± 1 m/s).
► Strong decrease in wind speeds above a latitude of 50°.
► SS–AS flow of 138 ± 4 m/s; no indication for a global RSZ component.
► Short term wind variabilities: changes up to 58 m/s within few days.