Longitude variation of ion temperature in the Io plasma torus

[1] The Io plasma torus shows an unanticipated variation of ion temperature as a function of System III (magnetic) longitude lambda(III). The simple expectation for the perpendicular temperature T-perpendicular to, based on ion pick-up in a tilted dipole magnetic field, is a double-peaked function with two maxima and two minima per 360degrees of longitude, having an average value of similar to 480 eV for S+ and a relatively small variation amplitude of similar to +/- 7 eV. Instead, the observed T-perpendicular to(S+) near Io's orbit has only one maximum and one minimum, with a much smaller average value of similar to 30 eV and a much larger relative variation amplitude of similar to +/- 50%. To explain the observed temperature variation, we propose that the longitude variation of the Pedersen conductance of Jupiter's ionosphere causes the pickup speed of ions injected near Io to vary with longitude. We obtain a satisfactory fit to the observed temperature variation if we make the straightforward assumption that the ionospheric conductance varies inversely with the strength of the ionospheric magnetic field. Both the magnitude and phase of the observed T-perpendicular to variation are consistent with this simple model. The observed parallel temperature T-parallel to(S+) has a somewhat smaller average magnitude and a similar longitude variation, but shifted about 70degrees in phase toward larger lambda(III). The T-parallel to behavior is consistent with the idea that T-parallel to derives mainly from T-perpendicular to through pitch-angle scattering in the presence of the observed corotation lag of torus plasma. The 70degrees phase shift requires a scattering timescale of similar to50 hours, consistent with the classical timescale for ion-ion Coulomb scattering.