Detection of Negative Pickup Ions at Saturn's Moon Dione

Abstract

We investigate a possible negative ion feature observed by the Cassini Plasma Spectrometer (CAPS) during a flyby of Saturn's moon Dione that occurred on 7 April 2010. By examining possible particle trajectories, we find that the observed particles are consistent with negative pickup ions originating near the moon's surface. We find that the mass of the negative pickup ions is in the range of 15-25 u and tentatively identify this species as O − , likely resulting from ionization and subsequent pickup from Dione's O 2 -CO 2 exosphere. Our estimates show that the negative ion density is~3 × 10 −3 cm −3 . This is comparable to, but slightly smaller than, that previously reported for the density of O 2 + pickup ions for the same flyby, indicating that negative pickup ions may represent a major loss channel for Dione's exosphere.

Photodetachment and Test-Particle Simulation Constraints on Negative Ions in Solar System Plasmas

“…Water-based negative ions and dust were similarly detected within the heart of the Enceladus plumes [Coates et al, 2010a,b, Jones et al, 2009 where they also carried the majority of the negative charge density [Morooka et al, 2011]. Further oxygen and possibly carbon-based negative ions populations have also been identified outflowing from Rhea [Teolis et al, 2010 and Dione [Nordheim et al, 2020. Negative ions and dust have also been shown to dominate in Saturn's deep ionosphere and, as at Titan and Enceladus, carry the majority of the negative charge and therefore the majority of the planet's ionospheric plasma mass density due to their large inferred masses [Morooka et al, 2019.…”

“…The densities of negative ions in the constrained range of 45-70 amu, likely comprising of water cluster ions, were detected at lower densities of 0.1 -1 cm −3 [Coates et al, 2010a]. These densities were however calculated using an instrument detection efficiency of 5 % and subsequent studies , Nordheim et al, 2020] use an increased rates of ≈20 %, derived from the studies of Peko and Stephen [2000] and Stephen and Peko [2000]. The negative ion densities are therefore likely lower than reported by Coates et al [2010a] but with large uncertainties.…”

“…Dione was discovered to possess a sputtered-induced oxygen and carbon dioxide exosphere . During D2, the second flyby of Dione, the Cassini spacecraft detected fluxes of outflowing negative ions within the moon's wake consistent with negative ions of masses 15-25 amu/q [Nordheim et al, 2020]. These were therefore identified as O − with densities of ≈ 3 × 10 −3 cm −3 , comparable to those of the positive ion outflows .…”

“…These negative ion detections were made just 1000 km downstream of the moon and therefore within the particles first gyration around the magnetic field. The particle trajectories shown in Figure 5 assume a Saturnian dipole field of ≈75 nT at Dione's orbit with particles spawned on Dione's surface where the detections have been identified as originating [Nordheim et al, 2020]. The particle trajectories show that negative oxygen ion fluxes will survive to nearly 15 Dione radii downstream before decreasing to 1 % of their source values.…”

“…McCarthy et al, 2006, Cordiner et al, 2013 and can outnumber free electrons in the Earth's D region [Appleton et al, 1933, Pavlov, 2014. The Giotto, Galileo, Cassini, Rosetta and Maven spacecraft have found evidence of negative ions existing in abundance within planetary and cometary plasmas with observations at 1P/Halley [Chaizy et al, 1991], Europa [Volwerk et al, 2001], Titan [Coates et al, 2007], Enceladus [Coates et al, 2010a], Rhea [Teolis et al, 2010], Dione [Nordheim et al, 2020], 67/P Churymov/Gerasimenko [Burch et al, 2015], Mars [Halekas et al, 2015] and Saturn's ionosphere [Morooka et al, 2019]. These observations were, however, serendipitously made with instruments neither designed nor calibrated for this purpose, and the extent to which negative ions pervade space plasmas is not well constrained.…”