ABSOLUTE WIND VELOCITIES IN THE LOWER THERMOSPHERE OF VENUS USING INFRARED HETERODYNE SPECTROSCOPY

Absolute wind velocities in the thermosphere of Venus were retrieved using NASA/Goddard Space Flight Center infrared heterodyne spectrometers at the NASA Infrared Telescope Facility (IRTF) and the McMath Solar Telescope, December 1985 to March 1987. Measurement of beam-integrated Doppler shifts in the nonthermal emission core of the 12C16O2 10.33-μm R(8) line provided sampling of the lower thermospheric wind field (100-120 km) projected along the line of sight. The spectrometer's 1-2 arc-sec diffraction-limited beam yielded the spatial resolution necessary for circulation model discrimination. Saturated resonance (Lamb dip) stabilization of the reference CO2 laser local oscillator provided continuous absolute frequency calibration of the spectrometer to <±0.1 MHz. Signal-to-noise limited line-of-sight velocity resolution to 2-5 m/sec was achieved. Qualitative analysis of beam-integrated winds indicated a dominant subsolar-to- antisolar circulation in the lower thermosphere. A retrograde zonal component was also suggested. Beam-integrated winds were modeled with a 100 × 100 element grid over the beam, incorporating beam spatial rolloff and across-the-beam gradients in nonthermal emission intensity, line-of-sight projection geometry, and horizontal wind velocity. Horizontal wind velocity was derived from a two-parameter model wind field comprised of subsolar- antisolar and zonal components. Acceptable model fits to the individual observing periods indicated a dominant subsolar-antisolar flow with 90-150 m/sec (cross-terminator) winds and a retrograde zonal component with a 15-45 m/sec (equatorial) velocity. No temporal variation in the wind field could be discerned at the sensitivity of the fits. A single, acceptable model fit was found common to all observing periods, with 120 m/sec subsolar-antisolar and 25 m/sec zonal retrograde components, indicating the wind field may have been constant December 1985 to March 1987. The 120 m/sec subsolar-antisolar component is consistent with the Bougher et al. (1986, 1988) two- and three-dimensional hydro- dynamical models for 110 km. © 1991.