Longitudinal asymmetry in the Io plasma torus.

A clear inbound-outbound asymmetry is found in Voyager 1 Low Energy Charged Particle (LECP) phase space densities near 6 R(J). The asymmetry is of the same sign and comparable magnitude for electrons and ions at several different energies. Ion phase space density profiles at three values of the first invariant are uniformly displaced to larger radius by ~ 0.25 R(J) during the inbound pass. Inbound-outbound asymmetries which are very similar for electrons and ions at many different energies can be accounted for naturally by a convection electric field in the plasma torus.