Energetic ion losses near Io's orbit

Abstract

We analyze energetic ion phase space densities obtained from the low energy charged particle experiment during the Voyager 1 inbound pass to Jupiter, near Io's orbit. We consider the radial transport of charged particles and the depletion of energetic ion fluxes there. Voyager 1 low energy ion data from the plasma experiment and ground‐based observations of the Io plasma torus are combined to determine both the absolute normalization and the functional form of the radial diffusion coefficient. This same diffusion coefficient is then used to infer the energetic ion loss rates from Voyager 1 low energy charged particle ion phase space densities. Energetic ion losses are observed in the region 5.9 ≤ L ≤ 7.1. The ion loss rates in this region are consistent with plasma wave scattering in the strong diffusion limit. The expected energy dependence in this limit is not found, however. There is evidence that the ion loss rate is not spatially uniform even within the main loss region. The rate of satellite sweeping is small compared to the observed loss rate, even in the Io sweeping corridor. The charge exchange loss rate is also small compared to the observed loss rate for L ≳ 5.9 and becomes important only for L < 5.5. Ion pitch angle distributions are presented, which support the idea that charge exchange losses of energetic ions become important within L < 5.5. This interpretation also implies that heavy ions contribute appreciably to energetic ion fluxes in that region.