Atmosphere of Venus as studied with the Mariner 5 dual radio-frequency occultation experiment

The amplitudes and differential Doppler of two radio signals transmitted to Mariner 5 as it was occulted by Venus are utilized to derive the dayside and nightside ionization distributions in the upper atmosphere, and the temperature and pressure profiles of the lower atmosphere. Both sides of Venus have ionization peaks near the 140-km altitude level; the day-time peak is 30 times greater in electron number density. The nightside ionosphere extended up to an altitude of at least 1000 km, and the dayside ionization terminated in a plasmapause near an altitude of 500 km. There may be a wake of planetary ionization extending a great distance in the antisolar direction. The neutral atmosphere was probed down to within 35 km of the planetary surface. Differences in the temperature and pressure profiles for the nightside and dayside, and apparent kilometer-size structure in the atmosphere, are discussed.