MEASUREMENT OF WINDS IN VENUS UPPER MESOSPHERE BASED ON DOPPLER SHIFTS OF THE 2.6-MM (CO)-C-12 LINE

Before inferior conjuction in 1988, we observed Venus at the first rotational transition of 12CO using the three-element, millimeter-wavelenght array at the Owens Valley Radio Astronomy Observatory and successfully detected winds in the upper mesosphere of Venus. The subearth point occurred at 19.3 hr local Venus time and the planet's disk showed the evening terminator. The 33″.4 diameter disk was resolved by a synthesized beam 2″.8 at full-width-half-maximum. The absorption line was finely sampled in frequency by a 32-channel filterbank with channel widths of 50 kHz. With this spatial and spectral resolution, it was possible to measure doppler shifts of the absorption line across the planet due to strong winds in Venus' upper mesosphere. Weighting functions show sampling of the mesosphere within a 12-km-wide layer centered at 97 km at normal incidence in the evening, 99 km on the dayside limb, and 103 km on the nightside limb. The doppler shifts change in a manner indicative of westward, horizontal winds-being strongly blue on the east/dayside limb, zero near the center, and strongly red on the west/nightside limb of the planet. Measurements of these shifts along the planet limb are closest to direct measurements of the horizontal speeds once corrected for incidence angle. The mean value along the dayside limb is 85 ± 8 m sec-1 and along the nightside limb is 106 ± 10 m sec-1. The radial wind speeds from the doppler shifts were smoothed to reduce noise and then fitted in a least-squares sense to canonical forms of the lower atmosphere's westward zonal flow and the thermosphere's subsolar-to-antisolar flow. The two flows are highly correlated in orientation in these observations of Venus' evening terminator. The smoothed measurements are best fitted for zonal flows in 10° latitude bins from 70°S to 61°N. The mean value of these fitted zonal flows is 132 ± 10 m sec-1. No facet of the data exists to firmly support or rule out the existence of a smaller (≤40 m sec-1) subsolar-to-antisolar flow in the upper mesosphere. © 1991.