Evidence of a source of energetic ions at Saturn

Abstract

We calculate phase space densities, in terms of adiabatic invariants of charged particle motion, using the low‐energy charged particle (LECP) data from the Voyager 2 encounter with Saturn. Ion data are corrected for electron contamination. For all energies considered here, phase space density at fixed first and second adiabatic invariant, ƒ(L), increases with increasing equatorial pitch angle. Evidence exists for a source of low‐energy ions (∼0.07‐4.2 MeV) and energetic protons (54‐152 MeV) close to Saturn. The source of >50 MeV protons is likely to be cosmic ray albedo neutron decay (CRAND), although we know of no source mechanism for the low‐energy ions. Low‐energy ion phase space densities calculated here have characteristics of solutions of a steady‐state radial diffusion equation with sources outside the region of interest and near Saturn and losses. Losses are attributed to satellites, rings, and Saturn's atmosphere and observed changes in ƒ with respect to the adiabatic invariants of motion are consistent with model loss rates in most cases. For energetic protons mirroring near the magnetic equator, phase space densities are in good qualitative agreement with other analyses of Pioneer 11 and Voyager 2 Cosmic Ray System (CRS) data indicating a likely CRAND source close to Saturn and absorption by Mimas. Voyager data phase space densities for CRAND protons that mirror off the magnetic equator are presented for the first time. An estimate of the upper limit on average G ring particulate size of ∼1 cm is described.