Results of 42 radio occultation measurements of the thermal structure of the Venus atmosphere at altitudes of 42 to 90 km, carried out with the help of Venera-15 and Venera-16 satellites during three occultation cycles from October 1983 to September 1984, are presented. The measurements were made in polar and near-polar regions of the southern and northern hemispheres, at latitudes from -89° to -60° and from 64° to 89° with solar zenith angles of 65° to 118°. The results point out the similarity of the thermal structures of both hemispheres. In particular, the southern hemisphere has the same cold collar region as the northern. The thermal structure of the atmosphere in both hemispheres depends mainly on the latitude and only weakly on solar illumination conditions. In the polar regions, at latitudes beyond 80°, the temperature profiles above the tropopause are close to isothermal, while in the mesosphere region of the near-polar atmosphere, there exists a large temperature inversion with a mean value of 19 K and a maximum of about 35 K. The transition from the near polar to polar regions in the northern atmosphere is characterized by rapid lowering of the tropopause altitude by 5 km, rising of temperature by 12 K, and rising of pressure by 170 mbar. In the southern hemisphere, the corresponding magnitudes change by 3.1 km, 11 K, and 86 mbar, respectively. There is a substantial difference between the polar regions at the tropopause level. Temperature and pressure at the tropopause in the southern polar atmosphere are 15 K and 68 mbar lower than in the Northern region. The thermal structure difference between the near-polar regions of the hemispheres is less significant. Comparison of these results with data of radio occultations obtained by the Pioneer Venus orbiter leads to the conclusion that the thermal structure of the atmosphere persists over a time interval on the order of 5 years. © 1991.
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