Cassini observations of the thermal plasma in the vicinity of Saturn's main rings and the F and G rings

Abstract

The ion mass spectrometer on Cassini detected enhanced ion flux near Saturn's main rings that is consistent with the presence of atomic and molecular oxygen ions in the thermal plasma. The ring “atmosphere” and “ionosphere” are likely produced by UV photosputtering of the icy rings and subsequent photoionization of O2. The identification of the O+ and O2+ ions is made using time‐of‐flight analysis and densities and temperatures are derived from the ion counting data. The ion temperatures over the main rings are a minimum near synchronous orbit and increase with radial distance from Saturn as expected from ion pick up in Saturn's magnetic field. The O2+ temperatures provide an estimate of the neutral O2 temperature over the main rings. The ion mass spectrometer also detected significant O2+ outside of the main rings, near the F ring. It is concluded that between the F and G rings, the heavy ion population most likely consists of an admixture of O2+ and water group ions O+, OH+, and H2O+.

Exospheres and Atmospheric Escape

“…The INMS (Ion Neutral Mass Spectrometer) and the CAPS (Cassini Plasma Spectrometer) experiments detected the presence of O + and O + 2 ions (Tokar et al 2005;Waite et al 2005b). A flux of thermal electrons (∼ 0.6-100 eV) was also detected and found to be in anti-correlation with the optical depth of the rings (Coates et al 2005), but the neutral atmosphere was below the INMS detection threshold (∼ 10 5 cm −3 ).…”

“…Therefore, the O + 2 detected at altitudes 0.1R S were scattered by charge exchange collisions with O 2 molecules . This process also injects O 2 into the Saturnian magnetosphere contributing to O + 2 detected outside of the ring system (Tokar et al 2005;Young et al 2005;Krimigis et al 2005). Therefore, the aeronomy occurring in Saturn's tenuous ring atmosphere could be the key to the understanding of an array of fundamental issues in the Saturnian system ranging from the large-scale structure of the ring system, the spoke phenomenon, the aeronomy of the Saturnian atmosphere, and the magnetospheric composition and dynamics.…”

How much oxygen is too much? Constraining Saturn's ring atmosphere

“…This Saturn Orbit Insertion (SOI) took Cassini at an altitude of ∼10,000 km over the main rings (see, e.g., Tokar et al, 2005), allowing unprecedented image resolution and opportunities for in situ plasma detection. These latter observations were carried out by the INMS and CAPS instruments, and made the exciting discovery of molecular and atomic oxygen ions (Waite et al, 2005;Young et al, 2005).…”

Ion Composition in Titan's Exosphere via the Cassini Plasma Spectrometer

“…This task will be accomplished in a straightforward manner with few modifications to the numerical tools that I have already developed; namely, the addition of a suitable model function. As mentioned in Section 3.1, Tokar et al [2005] used an isotropic, convective Maxwellian-so called because the Maxwellian is isotropic in a frame moving at the bulk flow velocity-to model water-group ions in SNG spectra acquired near Saturn's main rings. Similarly, Nemeth et al…”

“…The first involves direct integration of total ion counts over a partially sampled velocity distribution that is filled in by assuming symmetry in the reference frame of the flowing plasma [Sittler et al, 2005;Thomsen and Delapp, 2005;Sittler et al, 2006;Thomsen et al, 2010;Szego et al, 2011;and Szego et al, 2012]. The second method fits either isotropic or anisotropic Maxwell distributions to energy-and angle-discriminated ion counts in order to extract moments from the fit parameters [Tokar et al, 2005 andHill et al, 2008;Wilson et al, 2008;McAndrews et al, 2009;and Elrod et al, 2012]. In all of these cases the mass spectra are used primarily to determine relative abundances of H + , H 2 + , He ++ and the broad water group peak-consisting of unresolved O + , OH + , H 2 O + , and H 3 O + -in order to partition the total ion counts for moment calculations.…”