Solar Tides in the winds of the southern polar region of Venus using VIRTIS-M/Venus Express images

Abstract

The effect of the solar tides on the winds at the top of the clouds in Venus has been studied using cloud tracking technique applied to the Venus Express/VIRTIS-M images taken at wavelengths of 3.8 and 5.0 μm. Both these wavelengths probe about the same altitude on the clouds top, allowing for the first time to retrieve winds in the dayside and nightside simultaneously. The dataset included observations from 17 orbits, covering a time span of 290 days and a latitude range between 70ºS and 85ºS, a region where resides the so called cold collar. Both the diurnal (wavenumber 1) and the semidiurnal (wavenumber 2) tides are present, with the diurnal tide being the dominant harmonic for both the zonal and meridional components of the wind. The diurnal tide induces wind oscillations with amplitudes of about 4.5 m/s and 8.0 m/s for the zonal and meridional winds respectively. These amplitudes are in good accordance with the Rayleigh friction expected for this level of the Venus atmosphere, and support the important role of the diurnal tide in the maintenance of the mean zonal flow and in determining the sense of the meridional flow. While the tidal amplitude seems not to undergo important changes, the phase displays a temporal variability of about 1.4 hours in the local time coordinate. The rate of change of the phase seems different for the diurnal and semidiurnal component, indicative of a dispersive character of the solar tides, and is consistent with the expected change due to the tidal vertical propagation. Finally, a persistent lag is apparent in most cases between the tidal phases of zonal and meridional disturbances, implying that the diurnal tides tend to force equatorward winds when in the sense of the mean flow, and poleward winds when in the opposite sense.