Low‐energy plasma ion observations in Saturn's magnetosphere

We summarize observations of positive ions in Saturn's magnetosphere made by the Voyager plasma science experiments during the two Saturn encounters. In the outer magnetosphere, the number density and temperature of the plasma are quite variable. When cold, two ionic species are clearly resolved: the lighter ion is probably H + and the heavier ion has a mass/charge ratio consistent with it being O + or N + . In the vicinity of 15 Saturn radii ( R s ), there is a region in which the composition or charge state of the ions apparently changes, but it then returns to the typical composition. The two ionic components do not have the same thermal speeds or temperatures. In the outer magnetosphere, the two components partially corotate with the planet; their azimuthal speeds are approximately 20% below that expected for rigid corotation. There is some evidence for inward and outward radial flow in addition to the azimuthal motion in approximately one‐third of the Voyager 1 cold ion spectra. The sources of the plasma are still undetermined. The plasma sheet becomes well established within 16 R s from the planet in the Voyager 1 data and within 10 R s in the Voyager 2 data. Near the ring plane crossings close to Saturn, O + is the dominant ion within 1 R s of the ring plane, whereas H + dominates outside of that distance from the ring plane. Two scale heights characterize the O + ion number density distribution in the vicinity of the ring plane: within 0.5 R s of the plane, the scale height is ∼0.3 R s ; it is almost twice as great outside of 0.5 R s . The directly observed temperature of the O + ions is too high to be consistent with such scale heights. A temperature anisotropy of T ⊥ / T ∥ approximately equal to 6 would explain the observations. The cause of the anisotropy is not certain. In general, the plasma and magnetic field appear to be in dynamical equilibrium. In comparing Saturn's magnetosphere to Jupiter's, there appears to be much less acceleration of low‐energy plasma to higher energies at Saturn.