Hot ions in Jupiter's magnetodisc: A model for Voyager 2 low‐energy charged particle measurements

Abstract

The Low‐Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy‐dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two‐species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest‐energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near‐Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton‐Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower‐energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.