Structure of the ionosphere and atmosphere of Saturn From Pioneer 11 Saturn radio occultation

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° south and 11.6° 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 km. 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 km, with a broad peak of magnitude 7,000 cm−3 at an altitude of about 14,500 km, 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 km, if real, could be explained if densities of ∼105 exist 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%.