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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.

[1] Electron and ion drift dispersion events are often observed by the Cassini Plasma Spectrometer (CAPS) in the inner magnetosphere of Saturn (5 to 10 R S ). These events appear to result from azimuthally-limited injections of plasma and persist for at least several hours. During this time, the events can be analyzed to obtain information on the time and azimuthal location of the injections. The CAPS data show evidence of both remote and local injections. In this paper a conceptual model of Saturnian centrifugal interchange is developed based on the characteristics of the local injections.

The analysis of in situ plasma electron observations in the Io plasma torus by the plasma science experiment during the Voyager 1 encounter with Jupiter is presented in terms of two components: a thermal (c) Maxwellian component and suprathermal (H) non‐Maxwellian component of the electron distribution function. Average electron temperatures are Te < 1 eV in the cold torus (L < 5.5), with Te ≃ 5–6 eV in the hot torus (5.5 < L < 7.6); Te rises abruptly to Te ≃ 30 eV just outside the hot torus (L > 7.6) and then continues to rise to Te > 100 eV at r > 12 RJ. In the cold torus the density ratio of the suprathermal component nH to that of the cold component nc was <10−4; but in the hot torus, nH/nC ∼ 10−3 was observed, and outside the torus, nH/nc can exceed 10−1. We present evidence that suprathermal electrons are locally produced in the hot torus. Throughout the hot torus the electron temperature Te is a factor of 10 less than the thermal ion temperature. A large difference in the hot electron pressure PH is observed between the inbound and the outbound data which is interpreted as a latitudinal gradient with PH being a maximum at the magnetic equator. If one imposes the theoretical and observational constraint that (T⊥/T∥)EQ ≤ 2 for the hot electrons, then one requires the presence of a parallel electric field E∥ > 2.5 µV/m which exceeds the ambipolar electric field E∥ < 1 µV/m produced by the centrifugally confined ions. However, if unacceptable charge imbalances in the thermal plasma are not to occur from this larger E∥, then sufficient wave turbulence in the plasma must be present to adequately scatter the thermal electrons. We infer the presence of a neutral corona around Io from the observed decrease and symmetry with respect to Io of Tc. The energy input to the torus by charge exchange and ionization in this neutral corona followed by pickup is ∼2 × 1011 W, substantially less than the EUV luminosity. In the hot torus, suprathermal electrons contribute significantly to the ionization of the more highly ionized ions (O+, O2+, S2+, and S3+).

Extensive measurements of low-energy plasma electrons and positive ions were made during the Voyager 1 encounter with Saturn and its satellites. The magnetospheric plasma contains light and heavy ions, probably hydrogen and nitrogen or oxygen; at radial distances between 15 and 7 Saturn-radii (Rs) on the inbound trajectory, the plasma appears to corotate with a velocity within 20 percent of that expected for rigid corotation. The general morphology of Saturn's magnetosphere is well represented by a plasma sheet that extends from at least 5 to 17 Rs, is symmetrical with respect to Saturn's equatorial plane and rotation axis, and appears to be well ordered by the magnetic shell parameter L (which represents the equatorial distance of a magnetic field line measured in units of Rs). Within this general configuration, two distinct structures can be identified: a central plasma sheet observed from L = 5 to L = 8 in which the density decreases rapidly away from the equatorial plane, and a more extended structure from L = 7 to beyond 18 Rs in which the density profile is nearly flat for a distance +/- 1.8 Rs off the plane and falls rapidly thereafter. The encounter with Titan took place inside the magnetosphere. The data show a clear signature characteristic of the interaction between a subsonic corotating magnetospheric plasma and the atmospheric or ionospheric exosphere of Titan. Titan appears to be a significant source of ions for the outer magnetosphere. The locations of bow shock crossings observed inbound and outbound indicate that the shape of the Saturnian magnetosphere is similar to that of Earth and that the position of the stagnation point scales approximately as the inverse one-sixth power of the ram pressure.