Survey of ion plasma parameters in Saturn's magnetosphere

Abstract

[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 latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions

“…For each of these orbits we examined the radial component of the magnetic field (B r ), together with the azimuthal velocity and the density of the proton species derived by Thomsen et al (2010). Figure 1 shows two of these plots, for the T57 and T59 Titan encounters respectively.…”

“…The broad latitudinal coverage of these passes reveals the latitudinal structure of the flows in the outer magnetosphere together with a quasi-periodic variation associated with this structure. We investigate the azimuthal flow velocities and their periodicities using numerical ion moments (Thomsen et al, 2010) derived from the measurements of the Cassini Plasma Spectrometer (CAPS) onboard Cassini (Young et al, 2004). Our analysis covers data from the beginning of 2005 to the end of 2009, from southern summer to equinox conditions.…”

Saturn Plasma Sources and Associated Transport Processes

“…The almost co-rotating inner magnetosphere, which includes hydrogen ions likely to be mainly from Saturn's ionosphere, is dominated by waterbased neutrals (O, OH). Enceladus, supplemented by the rings and the associated neutrals, populates the outer magnetosphere as well (Smith et al, 2009, Thomsen et al, 2010. Some of the remarkably complex chemistry at Titan appears to involve particles, oxygen in particular, originally from Enceladus (Coates et al, 2007b, Sittler et al, 2009.…”

1. Transport of Mass, Momentum and Energy in Planetary Magnetodisc Regions

“…There is scope for improving the inner boundary conditions related to ion density in these multi-fluid calculations, in light of additional plasma moment determinations from the Cassini plasma spectrometer experiment (e.g. Thomsen et al 2010). The multi-fluid approach has the advantage over single-fluid MHD of being able to separately track the dynamics of the different ion species and sources, instead of simply calculating their average properties.…”