Thermal zonal wind in the Venus middle atmosphere according to Venera 15 IR-spectrometry

Abstract

The sounding geometry of Venera-15 allowed us to make measurements nearly along the meridian, and to obtain data practically simultaneously for a wide range of latitudes for almost the same local time. The results of detailed thermal wind field investigations, especially in the region of the mid-latitude jet, are presented. Vertical temperature profiles, retrieved from IR-spectra, are used to derive cyclostrophic zonal winds in the middle atmosphere of Venus (60–90 km, 30–70°N). The thermal wind velocity field in altitude-latitude coordinates has been obtained for each observing session. The typical distance between two measured spectra, used for vertical temperature retrieval, is 2–4 degrees of latitude at middle and high latitudes. The mid-latitude cloud top jet was revealed at about 67–70 km in most sessions. Its latitude changes from 42 to 58° N and its central velocity varies from 150 to 90 m/s. The jet wind velocity obtained at the morning terminator is about 35 m/s higher than that at the evening one. On the morning side, we found a correlation between the velocity and the latitude of the jet, which keeps the angular momentum conserved. A connection between the jet velocity and the cold collar is observed. The mid-latitude jet is usually not found when the cold collar is absent. From the unique session for the Southern hemisphere a South midlatitude jet at about 70 km altitude has been observed at 45° S, having the central velocity of 100 m/s. In addition, a low latitude jet was revealed near 80 km altitude at 15°S with a speed of 140 m/s. Fourier analysis of the jet velocity vs. solar longitude shows possible periods of about 60 days, 5.5 and 3.5 days. Two maxima of jet velocity have been obtained, at 9 AM (160 m/s) and 9 PM (130 m/s).