No abstract

During the flyby of Saturn by Pioneer 11 on September 1, 1979, radio occultation measurements of the ionosphere and upper neutral atmosphere were made near the terminator at latitudes 9.7 ø south and 11.6 ø south. The principal electron density peak of the ionosphere occurs at an altitude of about 1,800 km and has a magnitude of 11,400 cm -3, with a sharp lower peak of about 9,000 cm -3 at about 1,200 kin. The scale height above the main peak corresponds to an exosphere temperature of about 1150 K for an H + ionosphere. Ionization appears to extend to 30,000 kin, with a broad peak of magnitude 7,000 cm -3 at an altitude of about 14,500 kin, corresponding to the inner edge of the C ring. The low density of the lower portion of the ionosphere can probably be explained by ring shadowing and equatorial anomaly, and the relatively high densities about 30,000 kin, if real, could be explained if densities of ~ 103 e•ist at higher latitudes that are connected to the site of measurement by field lines and by plasmaspheric heating by the rings. In the neutral atmosphere, measurements were made to a pressure level of about 180 mbar, showing a temperature inversion region with a triple minimum. The temperature at the principal minimum at 60,344 km is 88 + 4 K at a pressure of 74 mbar, and there is a prominent minimum about 80 km above the principal one. Comparisons with the temperature structure derived from the infrared radiometer data (Ingersoll et al., this issue) suggest that the helium fraction in the Saturnian atmosphere is 10 + 4%. • to the spacecraft designers of the Pioneer project at NASA Ames Research Center and NASA/JPL (Jet Propulsion Laboratory) Deep Space Network, which made improvements in the ground receiving station radio systems that were more than sufficient to overcome the increased communication distanc•.A close flyby of Saturn necessarily brings the spacecraft into occultation by the planet and therefore provides an opportunity to probe its ionosphere and atmosphere with the signal from the spacecraft's communications link. The purpose of our experiment was to learn as much as possible about the structure of the electron density in the ionosphere and the temperature and pressure in the top layers of the neutral atmosphere within the limitations imposed by Pioneer's radio system and by the earth-Saturn-sun geometry at the time of encounter. The Pioneer 11 radio system employs a single S band (2.293 GHz) radio link for communication with the earth. Since only one frequency is available, the effects of charged particles cannot be isolated by means of the differen-tial Doppler technique, and so the frequency fluctuations caused by the ionosphere must be laboriously separated from the effects of oscillator drift, drifts introduced by orbital uncertainty, and the effects of interplanetary plasma fluctuations, which at the time of encounter were quite formidable because of the proximity of the earth-Saturn line to the sun. Spacecraft solar conjunction occurred only 10 days after encounter, and on encounter da...

Pioneer Venus orbiter dual-frequency radio occultation measurements have produced many electron density profiles of the nightside ionosphere of Venus. Thirty-six of these profiles, measured at solar zenith angles (chi) from 90.60 degrees to 163.5 degrees , are discussed here. In the "deep" nightside ionosphere (chi > 110 degrees ), the structure and magnitude of the ionization peak are highly variable; the mean peak electron density is 16,700 +/- 7,200 (standard deviation) per cubic centimeter. In contrast, the altitude of the peak remains fairly constant with a mean of 142.2 +/- 4.1 kilometers, virtually identical to the altitude of the main peak of the dayside terminator ionosphere. The variations in the peak ionization are not directly related to contemporal variations in the solar wind speed. It is shown that electron density distributions similar to those observed in both magnitude and structure can be produced by the precipitation on the nightside of Venus of electron fluxes of about 108 per square centimeter per second with energies less than 100 electron volts. This mechanism could very likely be responsible for the maintenance of the persistent nightside ionosphere of Venus, although transport processes may also be important.

No abstract

Radio occultation measurements at S band (2.293 gigahertz) of the ionosphere and upper neutral atmosphere of Saturn were obtained during the flyby of the Pioneer 11 Saturn spacecraft on 5 September 1979. Preliminary analysis of the occultation exit data taken at a latitude of 9.5 degrees S and a solar zenith angle of 90.6 degrees revealed the presence of a rather thin ionosphere, having a main peak electron density of about 9.4 x 10/(3) per cubic centimeter at an altitude of about 2800 above the level of a neutral number density of 10(19) per cubic centimeter and a lower peak of about 7 x 10(3) per cubic centimeter at 2200 kilometers. Data in the neutral atmosphere were obtained to a pressure level of about 120 millibars. The temperature structure derived from these data is consistent with the results of the Pioneer 11 Saturn infrared radiometer experiment (for a helium fraction of 15 percent) and with models derived from Earth-based observations for a helium fraction by number of about 4 to 10 percent. The helium fraction will be further defined by mutual iteration with the infrared radiometer team.