Observations of molecular oxygen ions in Saturn's inner magnetosphere

Abstract

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.

Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

“…Since sputtering and the Enceladus plumes primarily supply water products to the plasma (H + , O + , OH + and H 2 O + ), the ionization of neutral O 2 in the extended ring atmosphere is likely the primary source of O + 2 inside the orbit of Rhea. Therefore, plasma measurements of O + 2 give a clear marker for the extent of Saturn's ring atmosphere and the enhancement in O + 2 seen in the vicinity of Rhea can be attributed to a local source of O 2 (Martens et al 2008). Recent close flybys of Rhea have confirmed the presence of an atmosphere (Teolis et al 2010).…”

“…Therefore, they can be scattered into Saturn's atmosphere (Connerney and Waite 1984) or into the saturnian magnetosphere beyond the edge of the main rings forming an extended ring atmosphere. Ionisation of the O 2 in the extended ring atmosphere contributes to O + 2 detected outside of the ring system Martens et al 2008). Ignoring recycling of oxygen on the ring particle surfaces, Monte Carlo simulations ) indicate about 5 × 10 25 O s −1 are scattered out of the ring atmosphere for the illumination at SOI, either as O or O 2 .…”

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

“…Rhea's CO2 exosphere may therefore provide a source of the 44 u ions, or the carbon and oxygen ions via dissociative reactions, identified at radial distances of <20 RS [Christon et al, 2015]. If the breakup is sufficiently fast, this may provide some explanation for the elevated O + 2 levels reported by Martens et al [2008] at Rhea's orbit.…”

“…The magnetic signatures of mass loading have, however, not been reported in the vicinity of Rhea, despite increased O + 2 abundances being observed at these radial distances [Martens et al, 2008].…”

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

“…Using many Cassini orbits, Martens et al (2008) showed that molecular oxygen is a minor constituent of the magnetospheric plasma. Due to the low observed densities of this molecule, it is difficult to determine the source, or sources, of the corresponding ions.…”