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[1] A survey of the bulk plasma ion properties observed by the Cassini Plasma Spectrometer instrument over roughly the first 4.5 years of its mission in orbit around Saturn is presented. The moments (density, temperature, and flow velocity) of the plasma distributions below 50 keV have been computed by numerical integration of the observed counts in the "Singles" (non-mass-resolved) data, partitioned into species on the basis of concurrent determinations of the composition from the time-of-flight data. Moments are presented for three main species: H + , W + (water group ions), and ions with m/q = 2, which are presumed to be H 2 + . While the survey extends to radial distances of 30 R S and thus includes some solar wind or magnetosheath values, our principal interest is the large-scale spatial variation of the magnetospheric plasma properties, so we focus attention on radial distances inside of 17 R S . Principal findings include the following: (1) the densities of all three components are highly variable but are generally well organized by dipole L and magnetic latitude; (2) the density of ions with m/q = 2 varies from a few percentage of the H + density in the inner magnetosphere to a maximum of several tens of percentage near the orbit of Titan, suggesting that Titan is an important source for H 2 + in the outer magnetosphere; (3) water group ions are the dominant population in the inner magnetosphere, but only within ∼3 R S of the equatorial plane because of their strong centrifugal confinement; (4) derived latitudinal scale heights are largest for the light ions and generally increase with radial distance; (5) the L dependence of the calculated temperatures is not consistent with adiabatic transport but is in fair agreement with the expectations for plasma originating from ion pickup; (6) in agreement with the findings of Sergis et al. (2010), inside of L ∼ 11, the particle pressure is dominated by ions with energies below a few keV; (7) the derived flow velocities reveal the global circulation pattern of relatively dense populations in the magnetosphere, with no evidence for return circulation from the nightside to the dayside beyond ∼20 R S ; and (8) the azimuthal flow speeds are typically less than full corotation over the entire L range examined, varying from ∼50% to 70% of full corotation.

The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn's magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn's magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn's ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed

Electron and ion measurements made by the Voyager 1 plasma science instrument revealed a plasma wake surrounding Titan in Saturn's rotating magnetosphere. This wake is characterized by a plasma that is more dense and cooler than the surrounding subsonic magnetospheric plasma. The density enhancement is produced by the deflection of magnetospheric plasma around Titan and the addition of exospheric ions picked up by the rotating magnetosphere. By using simple models for ion pickup in the ion exosphere outside Titan's magnetic tail and ion flow within the boundaries of the tail, the interaction between Saturn's rotating magnetosphere and Titan is shown to resemble the interaction between the solar wind and Venus. Outside the magnetic tail of Titan, pickup of H+ formed by ionization of the H exosphere is indicated when synthetic and observed ion spectra are matched. Close to the boundary of the tail, a reduction in plasma flow speed is found, providing evidence for mass loading by the addition of N2+/H2CN+ and N+ to the flowing plasma. The boundary of the tail is indicated by a sharp reduction in the flux of high‐energy electrons, which are removed by inelastic scattering with the atmosphere and centrifugal drift produced when the electrons traverse the magnetic field draped around Titan. Within the tail the plasma is structured as the result of spatial and/or temporal variations. The ion mass cannot be determined uniquely in the tail; however, one measurement suggests the presence of a heavy ion with a mass of order 28 amu: One candidate is H2CN+, suggested as the dominant topside ion of the ionosphere, which may flow from the ionosphere into the tail.

For reference purposes we have superimposed dipole field lines in Figure 1b. There are corrections to Saturn's dipole field, as evolving model calculations by Connerney et al. (1981, 1Sa82) suggest the presence of quadrupole and octupole terms in the internal field and a ring current between Lx8.5 and 15.5; these corrections become important outside L=8, where the ring current produces an inflation of the dipole field lines.The lack of tilt in most magnetic field models makes the spin equator nearly congruent with the magnetic equator. This has the unfortunate consequence that each spacecraft does not make as many crossings of Saturn's plasma sheet during an enc;.Lmter as was the case during the Jupiter encounters. This symmetry, however, produces a simplification in our interpretation of the plasma data, as it makes the centrifugal and magnetic equatorial planes nearly coincident. Under this condition, the plasma, regardless of its thermal characteristics, will have mirror symmetry about the equatorial plane. As noted in Smith et al. (1980), the corotational electric field can dominate the convective electric field due to the solar wind out to radial distances in excess of 21 R S , the average radial position of the noon time magnetopause boundary. In first approximation, one expects Saturn's magnetosphere to be azimuthally symmetric inside L05; the magnetic field data reported by Smith et al. (1980) and Ness et al. ( , 1982 support this expectation. Finally, using the above approximations of dipole field, mirror symmetry, azimuthal symmetry, and making the additional assumption of "steady-state" one can increase the coverage of the spatial distribution of the plasma in L, Z space by combining the plasma data from all three encounters. In this way, Bridge et al. (1982) were able to construct a fairly extensive description of the plasma morphology.