Energetic ion phase space densities in Neptune's magnetosphere

Abstract

Ion intensities measured by the Voyager 2 Low Energy Charged Particle (LECP) experiment at Neptune have been analyzed to determine ion phase space densities at fixed values of the first and second adiabatic invariants of charged particle motion. The phase space densities are broadly peaked near L = 10 and generally show a rapid decline going toward Neptune, although there is a gap in data coverage at the Proteus (1989N1) minimum L-shell. These profiles are interpreted as indicating generally inward radial diffusion with an energetic ion source near L = 10. There is excellent agreement between inbound and outbound phase space density profiles at the same values of the invariants, suggesting quasistationary and roughly axisymmetric radiation belts. If absorption by Neptune's moons and rings is an important loss process, then the radial diffusion coefficient D LL is on the order of 10 −7 L 3 sec −1, consistent with the Voyager Plasma Science determination for a Triton-associated proton source of 10 25 sec −1. The LECP ion phase space density profiles are consistent with relatively weak L-dependence of D LL , provided that loss rates increase toward the planet; for an extreme case of a loss rate independent of L, D LL ∝ L 6.03. Weaker L dependence of D LL is found for a loss rate τ −1 increasing toward the planet ( D LL ∝ L 3 when τ −1 ∝ L −3.63), suggesting interchange diffusion. With D LL = 6 × 10 −8 L 3 sec −1, the inward diffusing power carried by energetic ions is estimated as 2 × 10 9 W, which would be adequate to power Neptune's aurora if a substantial portion of the ions is lost into Neptune's atmosphere.