Positive ion observations in the middle magnetosphere of Jupiter

Abstract

We consider the positive ion data gathered by the Voyager Plasma Science experiment in the middle magnetosphere of Jupiter. The experiment measures positive ions with energies per charge between 10 and 5950 V. The observations are analyzed to obtain the mass and charge densities, velocity components, and temperatures of the low‐energy plasma population. The reduced data set is discussed in the context of the outstanding questions concerning this plasma population and its dynamics. We find that on the dayside, there exists a transonic to highly supersonic positive ion population which tends to move azimuthally but does not rigidly corotate with the planet. These ions provide the inertia of the magnetospheric plasma inside of ∼40 RJ. The mass density is everywhere dominated by heavy ions, and the mass density gradient is consistent with outward diffusion from the Io plasma torus via flux tube interchange. The ions tend to be concentrated in a plasma sheet which is associated with the current sheet inferred from the magnetic field observations. The plasma in the sheet is relatively cool (∼20 eV) in comparison with plasma at higher magnetic latitudes (≳100 eV). In addition to the azimuthal flow pattern, we find a local time asymmetry in the data which we interpret as flow away from the current sheet on the dayside and toward the current sheet on the nightside. This dynamic expansion and contraction of the plasma sheet is presumably driven by the asymmetry in the magnetosphere due to the solar wind interaction.