Absolute wind measurements in the lower thermosphere of Venus using infrared heterodyne spectroscopy

The first absolute wind velocities above the cloud-tops were obtained using NASA/Goddard Space Flight Center infrared heterodyne spectrometers at the NASA IRTF and the McMath Solar Telescope, December 1985 to March 1987, during four planetary phases spanning 82% of a synodic period. Bean-integrated Doppler displacements in the non-thermal emission core of $\sp{12}{\rm C}\sp{16}{\rm O}\sb2$ 10.33 $\mu$m R(8) (obtained at sub-Doppler resolution: $\lambda/\Delta\lambda$ = 10$\sp7$), sampled the line of sight projection of the lower thermospheric wind field (100-120 km). A field-usable Lamb-dip laser stabilization system, developed for spectrometer absolute frequency calibration to $<$ $\pm$ 0.1 MHz, allowed S/N-limited line of sight velocity resolution at the 1 m/s level. The spectrometer's diffraction-limited beam (1.7 arc-second HPBW at McMath, 0.9 arc-second HPBW at IRTF), and 1-2 arc-second seeing, provided the spatial resolution necessary for circulation model discrimination. Qualitative analysis of beam-integrated winds definitively established a dominant subsolar-antisolar circulation in the lower thermosphere. A retrograde zonal component was also suggested. Beam-integrated winds were modelled with a 100 x 100 grid over the beam, incorporating beam spatial rolloff and across-the-beam gradients in non-thermal emission intensity, line of sight projection geometry, and horizontal wind velocity. Horizontal wind velocity was derived from a 2-parameter model wind field comprised of subsolar-antisolar and zonal components. Best-fit models indicated a dominant subsolar-antisolar flow with 120 m/s cross-terminator winds and a retrograde zonal component with a 25 m/s equatorial velocity (assuming wind field temporal stability over the synodic period). The subsolar-antisolar component is consistent with the Bougher et al. (1986) 2-dimensional hydrodynamical model for 110 km. Detection of a significant zonal component indicates upper thermospheric super-rotation may be coupled to, hence pumped by, cloud level super-rotation. Early heterodyne mesospheric wind measurements (Betz et al. 1977b) were re-examined based on laser stability test results. A reinterpretation is consistent with superposition of a 94 $\pm$ 6 m/s zonal super-rotation and a 35 $\pm$ 6 m/s antisolar-subsolar return flow at $\sim$75 km. A review of all dynamical indicators above the cloud-tops allowed development of an integrated and self-consistent picture of circulation in the 70-200 km range. Finally, historical visual observations of solar phase-locked structure were reviewed and discussed in light of the heterodyne results.