MAGELLAN RADIO OCCULTATION MEASUREMENTS OF ATMOSPHERIC WAVES ON VENUS

Radio occultation experiments were conducted at Venus on three consecu tive orbits of the Magellan spacecraft in October 1991. Each occultati on occurred over the same topography (67 degrees N, 127 degrees E) and at the same local time (22(h)5(m)), but the data are sensitive to zon al variations because the atmosphere rotates significantly during one orbit. Through comparisons between observations and predictions of sta ndard wave theory, we have demonstrated that small-scale oscillations in retrieved temperature profiles as well as scintillations in receive d signal intensity are caused by a spectrum of vertically propagating internal gravity waves. There is a strong similarity between the inten sity scintillations observed here and previous measurements, which per tain to a wide range of locations and experiment dates. This implies t hat the same basic phenomenon underlies all the observations and hence that gravity waves are a persistent, global feature of Venus' atmosph ere. We obtained a fairly complete characterization of a gravity wave that appears above the middle cloud in temperature measurements on all three orbits. The amplitude and vertical wavelength are about 4 K and 2.5 km, respectively, at 65 km. A model for radiative damping implies that the wave intrinsic frequency is similar to 2 x 10(-4) rad sec(-1 ); the corresponding ratio between horizontal and vertical wavelengths is similar to 100. The wave is nearly stationary relative to the surf ace or the Sun. Radiative attenuation limits the wave amplitude at alt itudes above similar to 65 km, leading to wave drag on the mean zonal winds of about +0.4 m sec(-1) per day (eastward). The sign, magnitude, and location of this forcing suggest a possible role in explaining th e decrease with height in the zonal wind speed that is believed to occ ur above the cloud tops. Temperature oscillations with larger vertical wavelengths (5-10 km) were also observed on all three orbits, but we are unable to interpret these unambiguously. (C) 1995 Academic Press, Inc.