Anisotropy and proton density in the Io plasma torus derived from whistler wave dispersion

During the Voyager 1 encounter with Jupiter, a large number of whistler waves were observed. Previous studies have examined the dispersion of these waves and made estimates of the electron and light ion (i.e., proton) densities. The current paper reexamines this data, taking into account the revised temperatures of the torus species the additional data on ion composition from the Voyager UVS instrument and the role of thermal anisotropy on the plasma densities. These refinements in the density model drastically alter the implications of the whistler wave data. Both the thermal and the nonthermal species must be anisotropic to fit the whistler dispersions. The thermal component must have T⊥/T‖ > 1.75 and the nonthermal component 3 < T⊥/T‖ < 10, The equatorial proton density is low, under 60 cm−3 in all cases. This results in a proton abundance (L shell proton content relative to the total ion content) of no more than 10%, approximately a factor of two lower than the conclusions of previous whistler analysis. At the high latitudes, the implied electron density results in a plasma frequency of under 20 kHz. Finally, it is evident from this analysis that not all of the whistler waves were propagating along the magnetic field lines, as was commonly assumed in previous work.