Sources and sinks of energetic electrons and protons in Saturn's magnetosphere

Abstract

This paper reports the results of continuing analysis and interpretation of energetic particle observations obtained by the University of Iowa instrument on Pioneer 11 during traversal of Saturn's magnetosphere in August–September 1979. On the basis of the radial dependence of the phase space density of very energetic protons (Ep >80 MeV) and estimates of the necessary source strength, it is reasonably certain that cosmic ray neutron albedo from the planet's atmosphere and Rings A and B is the source of such particles. At radial distance r=2.7 Rs and for Ep >80 MeV, the ratio of source strength 𝒮 to radial diffusion coefficient D is ∼2 × 10−24 cm−5. A reasonable pair of values is 𝒮 ∼7 × 10−15 cm−3 s−1 and D ∼1 × 10−10 Rs² s−1. The spectrum of electrons 0.040< Ee <(few MeV) and the radial dependence of the phase space density of such electrons are derived. The source of these electrons is at the magnetosheath. The source of protons Ep ∼1 MeV is also at the magnetosheath and may be either thermalized solar wind or energetic interplanetary protons which were unusually abundant at the time of Pioneer 11's encounter. Low energy electrons and protons experience strong losses inward of r ∼10 Rs, apparently attributable to the large inner satellites Rhea, Dione, Tethys, Enceladus, and Mimas and to a broad tenuous ring of particulate matter (Ring E) and/or gas. There were important temporal changes in the distributions of low energy electrons and protons within a period of ∼10 hours between inbound and outbound passes, probably caused by unusually disturbed interplanetary conditions.