Measurements of the mean, solar‐fixed temperature and cloud structure of the middle atmosphere of Venus

Data from the orbiter infrared radiometer (OIR) of the Pioneer Venus mission have allowed the global structure of the middle atmosphere of Venus to be studied in detail for the first time. Between 4 December 1978 and 14 February 1979 this instrument made over 300 000 soundings in ten spectral channels, covering most of the northern and some of the southern hemisphere in the altitude range 65–100 km (100–0·01 mb). Preliminary analyses indicate that mean atmospheric structure in this region is strongly dependent on latitude and local time of day, although day-to-day variations are seen. This paper presents the mean temperature field and cloud top structure retrieved from the data of five OIR channels averaged in a solar-fixed coordinate system. The retrieval scheme and the derivation of the weighting functions that characterize the vertical response of each channel are also described. The middle atmosphere can be divided into two distinct regions separated by a low-latitude temperature minimum of less than 170K at 95km in the retrieved zonal-mean temperature field. Below 95km, day-night temperature contrasts are small but above 70km pole-equator contrasts are positive reaching a maximum of 20–25 K at 85km. Fourier analysis shows that within 45° of the equator the longitudinal variation of temperature is dominated by wavenumber-2 tidal structure with a phase that moves eastwards with increasing altitude. Above 95km, the pole-equator temperature gradient is reversed, day-night contrasts become appreciable and wavenumber-1 longitudinal structure dominates. At equatorial latitudes mean cloud optical depth at 11·5 μn is unity at 100mb (66·5 km), and the cloud top has a scale height of 0·85 times the atmospheric value. The cloud top falls slowly with increasing latitude and has a wavenumber-1 longitudinal dependence of £20mb (£1 km), with the highest cloud found just before the evening terminator. Retrievals are insensitive to cloud structure in the polar regions, but it is clear that the cool collar that surrounds the warm polar region is not a high-cloud feature. It is in fact a deep temperature inversion in which temperatures can be more than 30 K less than equatorial values at the same level.