The solar wind plasma analyzer on board Pioneer 11 provides first observations of low‐energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy per unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range ∼4–16 Rs in the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10 Rs. At radial distances beyond 10 Rs, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8 Rs, the dominant ions are most likely protons, and the corresponding typical densities and temperatures are 0.5 cm−3 and 106 °K, respectively, with substantial fluctuations. Identification of the mass per unit charge (M/Q) of the dominant ion species is possible in certain regions of Saturn’s magnetosphere via the angular distributions of positive ions. A large torus of oxygen ions is located inside the orbit of Rhea, and the densities are >10 cm−3 over the radial distance range ∼4–7.5 Rs. Density maxima appear at the orbits of Dione and Tethys where oxygen ion densities are ∼50 cm−3. The dominant oxygen charge states are O2+ and O3+ in the radial distance ranges ∼4–7 Rs and 7–8 Rs, respectively. The observations are suggestive of a decrease of ion energies to values less than the instrument energy threshold of E/Q=100 eV at the apparent inward edge of the torus at 4 Rs. Ion temperatures increase rapidly from ∼2 × 105 °K at 4 Rs to 5 × 106 °K at 7.3 Rs. It is concluded that the most likely source of these plasmas is the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The sources associated with the satellites Dione and Tethys are probably of lesser strength. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere, since the pressure ratio β of these plasmas approaches unity at radial distances as close to the planet as 6.5 Rs. On the basis of these observational evidences it is anticipated that quasi‐periodic outward flows of plasma, accompanied by a reconfiguration of the magnetosphere beyond ∼6.5 Rs, will occur in the local night sector in order to relieve the plasma pressure from accretion of plasma from the rings.

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.

Intense electrostatic waves beyond the plasmapause have recently been identified at frequencies near the upper hybrid resonance frequency. In addition, the waves occur within a band at an odd, half‐harmonic of the local electron gyrofrequency. These bands of electrostatic turbulence are among the most intense waves detected within the earth’s magnetosphere. Measurements obtained with the ISEE 1 plasma wave receiver show that the intense waves appear to be intensifications of an electrostatic cyclotron harmonic band near the upper hybrid resonance frequency. A straightforward explanation of intense waves at the upper hybrid resonance frequency exists in the electrostatic multi‐cyclotron emission theory. For a broad range of plasma parameters nonconvective instability or large spatial growth rates occur within the cyclotron band encompassing the cold upper hybrid frequency. Comparison of spatial growth rate spectra with measured wave spectra shows that there is excellent qualitative agreement between the linear theory and the observed wave characteristics.

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Significant progress has been made in understanding intense electrostatic waves near the upper hybrid resonance frequency in terms of the theory of multiharmonic cyclotron emissions using a classical loss‐cone distribution function as a model. Recent observations by Hawkeye 1 and GEOS 1 have verified the existence of loss‐cone distributions in association with the intense electrostatic wave events, however, other observations by Hawkeye and ISEE have indicated that loss cones are not always observable during the wave events, and in fact other forms of free energy may also be responsible for the instability. Now, for the first time, a positively sloped feature in the perpendicular distribution function has been uniquely identified with intense electrostatic wave activity. Correspondingly, we suggest that the theory is flexible under substantial modifications of the model distribution function.