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The Composition of ~96 keV W+ in Saturn’s Magnetosphere
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  • Stephen Christon,
  • Robert DiFabio,
  • Douglas Hamilton,
  • Donald Mitchell,
  • Tom Krimigis
Stephen Christon
Focused Analysis & Research, Focused Analysis & Research
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Robert DiFabio
University of Maryland. University of Louisiana, University of Maryland. University of Louisiana
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Douglas Hamilton
University of Maryland, University of Maryland

Corresponding Author:dch@umd.edu

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Donald Mitchell
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Tom Krimigis
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The plumes of Enceladus produce a cloud of neutral H2O molecules and, via dissociation, OH and O. These neutrals are ionized by charge exchange, solar UV, and electron impacts, producing the thermal water group ions W+ (O+, OH+, H2O+, and H3O+) which become energized in Saturn’s magnetosphere. We first separate the components of energetic (~96 keV) W+ using Cassini Charge-Energy-Mass Spectrometer (CHEMS) data from 78 near equatorial main ring current passes (dipole L = 7-16, ±10° in latitude) in 2004-2010. We find ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+ when averaged over L = 7-16, resulting in a mean water group mass of 16.7 amu. At 7 < L < 21, we find abundance ratios for O+/W+, OH+/W+, and H2O+/W+ that vary little with L. However, while H3O+/W+ is nearly constant at L > 13, H3O+/W+ tends to increase persistently at L < ~10. The large O+ abundance qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Our observation of H2O+/W+ > ~20% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow H2O torus centered on Enceladus’ orbit.
Aug 2020Published in Journal of Geophysical Research: Space Physics volume 125 issue 8. 10.1029/2019JA027315