Although the first direct detection of the presence of plasmas in the vicinity of Io's orbit was reported by Frank et al. (1976) with measurements from the plasma analyzer on Pioneer 10, the first definitive measurements of Jovian magnetospheric plasmas were acquired during the Voyager flyby. The Voyager plasma observations were used to define the required capabilities for the Galileo plasma instrumentation. Briefly we summarize here the plasma domains of the Jovian magnetosphere. This information is largely taken from the review by Belcher (1983). For more recent work the reader is referred to further analysis of the torus ions (Bagenal, 1985; Bagenal et al., 1985), the torus electrons (Sittler and Strobel, 1987), and the middle magnetosphere (Sands and McNutt, 1988). Measurements of medium-energy charged particles, E > 30 keV, are summarized by Krimigis and Roelof (1983).

The heart of the Jovian magnetosphere is the great torus of plasmas that encompasses the orbit of Io. This torus is fed by the ionization of neutral gases from Io's atmosphere and may respond to the sporadic injection of gases from this moon's volcanic activity. The composition of the ion plasmas in this torus is rich in heavy ions, e.g., S+, O+, S2+, O2+, and Na+. The plasma torus is divided into two regimes, a cold torus inside Io's orbit and a hot torus at greater Jovicentric distances. The maximum ion densities, ~3,000 cm-3, are located near Io's orbit. The ion temperature decreases severely from ~40 eV at 6 RJ (Jovian radii) to ~1 eV at 5 RJ. This temperature decrease is due to radiative cooling. Ion temperatures in the hot torus at radial distances ~6 to 8 RJ are in the range of 40 to 100 eV. The electron temperatures can be described in terms of a two-temperature Maxwellian distribution, i.e., a cold and hot distribution. At the inner edge of the torus the electron temperatures decrease to ~0.5 eV with decreasing radial distances whereas the cold electron temperatures beyond ~6 RJ are typically ~10 to 100 eV. Characteristic temperatures of the hot electron velocity distributions are ~1 keV and the number densities are less than those for the cold electrons. The torus plasmas corotate with the planet. The corresponding corotational energy of an S+ ion is 960 eV at equatorial radial distance 6 RJ. The deflection of plasma bulk flow near the Io flux tube is consistent with that expected for incompressible flow around a cylinder and is evidence for an Alfven wave associated with the plasma flow past Io. The estimated current in the Io flux tube is ~3 × 10^6 amperes, presumably carried in large part by electrons.

At distances beyond the plasma torus, > 10 RJ, a plasma sheet extends to the dayside magnetopause. At 15 RJ the typical thickness of the plasma sheet is ~2 RJ. These plasmas are observed to corotate more-or-less rigidly with Jupiter's rotational motion to radial distances of about 20 RJ. At distances of 20 to 40 RJ this azimuthal bulk speed of the plasma is less than that expected from rigid corotation by factors of 2 or more. Beyond 40 RJ the plasmas are again observed to rigidly corotate at frequent times as inferred from measurements with the medium-energy charged particle detector. The corotational energy of an S+ ion at 40 RJ is 43 keV. Whereas the density of the hot electrons is only ~1% of the total density at 8 RJ, the hot electron density is similar to that for the cold electrons at 40 RJ. Ion temperatures are also higher in the plasma sheet relative to those in the torus, ~20 to 40 keV at radial distances 30 to 100 RJ. Plasma densities in the plasma sheet are ~1 to 10 cm-3 at 10 to 20 RJ and vary from ~10-3 to 1 cm-3 at larger radial distances. Above and below the plasma sheet the densities can be as low as 10-5 to 10-4 cm-3.

Beyond radial distances of 130 RJ in the dawn side of the Jovian magnetosphere the ion bulk flows become generally antisunward with a strong component along directions that are radially outward from the planet. This region was detected with the medium-energy charged particle detector and is called the magnetospheric wind.