HOT IONS IN JUPITERS MAGNETODISC - A MODEL FOR VOYAGER-2 LOW-ENERGY CHARGED-PARTICLE MEASUREMENTS

The Low-Energy Charged Particle (LECP) instrument on the Voyager 2 spa cecraft acquired a comprehensive set of directional and energy-depende nt information on the nature of hot ions in the Jovian magnetodisc. Th e 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 kappa distribution function model for h ot ions in the Jovian magnetodisc in the vicinity of neutral sheet cro ssings. The regions where-the model could be used ranged from 60 to 30 R(J) on the dayside (inbound) and 75 to 125 R(J) on the nightside (ou tbound). With this model, the full angular and spectral information fr om the lowest-energy LECP detectors has been deconvolved using a nonli near least squares technique to reveal the heavy ion pressure, density , and temperature distinct from the corresponding hot proton parameter s. 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 di stinct increase with radial distance paralleling the corotation or pic kup energy of heavy ions. A neutral wind of heavy atoms, originating i n the near-Io regions and ionized during their flight through the oute r magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasm a is subcorotational, reduced from the rigid value by a factor of simi lar to 2, but increases with increasing distance from 30 to 60 R(J) in the dayside region and from 75 to 85 R(J) in the nightside region. Th e trend stops beyond 85 R(J) in the nightside region, but there is sti ll a substantial corotational flow that extends from 85 R(J) to at lea st 130 R(J). In all the regions studied, the particle anisotropies in the LECP scan plane below similar to 2 MeV are believed to result prim arily from the Compton-Getting effect and not from gradient anisotropi es or particles executing nonadiabatic orbits as they encounter the ne utral sheet. Gradient anisotropies are not important even in the dista nt nightside neutral sheet region (> 85 R(J)) below similar to 2 MeV. The large flow velocities and increasing heavy ion temperatures are co nsistent with a strong corotational electric field and imply that the mass loading due to lower-energy heavy ion plasma via outward transpor t from Io is insufficient to disrupt corotation within similar to 60 R (J) during the Voyager 2 encounter.