[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.
We present an analysis of molecular oxygen ion (O2+) abundance in Saturn's inner magnetosphere based on observations made with the Cassini Plasma Spectrometer (CAPS). Using data summed over 23 orbits, we resolve and isolate O2+ counts from the tail of the water group ion mass distribution (W+ [O+, OH+, H2O+, H3O+]) and from background sources. O2+ was initially estimated at ∼1–2% of the total ion population in the inner magnetosphere based on CAPS data from the Saturn orbital insertion pass. Through refined analysis techniques, we have found O2+ to account for just above 0.3% of all W+ ions at L = 4.5, with only minor fluctuations in relative density out to L = 7.5 − 8.0. Beyond L = 7.5, the relative O2+/W+ abundance exhibits a statistically significant increase through L = 10.5, which may indicate a neutral O2 source in the vicinity of Rhea.
[1] We report observations of electric field solitary structures, measured by the Cassini Radio and Plasma Wave Science (RPWS) instrument in the vicinity of Saturn's magnetosphere with ambient magnetic fields that range from $0.1 nT to 8000 nT. The peak-to-peak electric field amplitudes of the observed solitary structures range from a few mV/m to 10 mV/m and show a slight trend toward larger amplitude electric field pulses being associated with larger ambient magnetic fields. The time durations of the pulses range from a few hundred ms to a few 10's of milliseconds. The solitary waves appear in plasma boundary regions or in regions with abrupt changes in the magnetic field. The solitary waves tend to be observed when the dipole antenna is aligned with the magnetic field. Citation: Williams, J.
A novel method to calculate the neoclassical radial electric field in stellarator plasmas is described. The method, which does not have the inconvenient of large statistical fluctuations (noise) of standard Monte Carlo technique, is based on the variation of the combined parallel and perpendicular pressures on a magnetic surface. Using a three-dimensional gyro-kinetic δf code, the calculation of the radial electric field (E r ) in the National Compact Stellarator Experiment has been carried out. It is shown that a direct evaluation of E r based on a direct calculation of the radial particle flux is not tractable due to the considerable noise.
[1] We document the presence of solitary structures in the electric field, measured by the Cassini plasma wave instrument at an interplanetary shock associated with the October/November 2003 solar flares. The occurrence frequency of electrostatic solitary waves increases prior to and during the passage of the initial shock boundary but decreases to almost zero in the post-shock environment. The electric field amplitudes of the solitary structures are on the order of a few tens of mV/m, while the characteristic scale size is estimated to be $500 Debye lengths. The estimated potentials are $0.5 V both upstream and downstream of the shock. These measurements present a new plasma regime which support electrostatic solitary structures. Citation: Williams, J.
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