Saturnian Trapped Radiation and Its Absorption by Satellites and Rings: The First Results from Pioneer 11

Abstract

Electrons and protons accelerated and trapped in a Saturnian magnetic field have been found by the University of Chicago experiments on Pioneer 11 within 20 Saturn radii (R_s) of the planet. In the innermost regions, strong absorption effects due to satellites and ring material were observed, and from ∼ 4 R_s inwards to the outer edge of the A ring at 2.30 R_s (where the radiation is absorbed), the intensity distributions of protons (≥ 0.5 million electron volts) and electrons (2 to 20 million electron volts) were axially symmetric, consistent with a centered dipole aligned with the planetary rotation axis. The maximum fluxes observed for protons (> 35 million electron volts and for electrons < 3.4 million electron volts) were 3 x 10⁴ and 3 x 10⁶ per square centimeter per second, respectively. Absorption of radiation by Mimas provides a means of estimating the radial diffusion coefficient for charged particle transport. However, the rapid flux increases observed between absorption features raise new questions concerning the physics of charged particle transport and acceleration. An absorption feature near 2.5 R_s has led to the discovery of a previously unknown satellite with a diameter of ≈ 200 kilometers, semimajor axis of 2.51 R_s, and eccentricity of 0.013. Radiation absorption features that suggest a nonuniform distribution of matter around Saturn have also been found from 2.34 to 2.36 R_s, near the position of the F ring discovered by the Pioneer imaging experiment. Beneath the A, B, and C rings we continued to observe a low flux of high-energy electrons. We conclude that the inner Saturn magnetosphere, because of its near-axial symmetry and the many discrete radiation absorption regions, offers a unique opportunity to study the acceleration and transport of charged particles in a planetary magnetic field.