Spatial and Temporal Variations of the Suprathermal (3-220 keV/e) Ion Composition in Saturn's Equatorial Magnetosphere

Abstract

We have studied the composition of the suprathermal ions (3-220 keV/e) in Saturn's magnetosphere using data from the Charge-Energy-Mass Spectrometer (CHEMS) on the Cassini Saturn orbiter from 78 equatorial (Latitude = -10° to 10°) passes during the period 2004-2010. We use this data to determine how the suprathermal ion density and composition vary with time, energy, and distance from Saturn (L-shell). This information is used to study the relative importance of various sources of plasma and neutral particles and how they may vary with time. We also examine the loss and energization processes of the suprathermal ions. In Saturn's main ring current region (L=7-16), we find that the suprathermal ion composition is 63% W+ (O+, OH+, H2O+, H3O+), 30% H+, 5.2% H2+, 0.98% O++, 0.55% He+, and 0.10% He++. The C+/W+ and N+/W+ ratios are 0.0055 and 0.0078, respectively. The high abundance of W+ indicates that the water plumes of Enceladus are the largest source of neutrals that, after ionization, become the plasma in Saturn's magnetosphere. Saturn's atmosphere, thought to be the strongest source of neutral H, and Titan's atmosphere, thought to be the strongest source of neutral H2, are also important. The low abundance of He++ and He+ indicates that the solar wind is a relatively minor source of plasma to Saturn's magnetosphere. The long-term time variations of W+, H+, and H2+ over the period 2004-2010 are quite modest while those of He++ and He+ are much larger. The small variation of the W+ indicates that the strength of the Enceladus plumes is relatively constant on time scales of months or longer over this period. The partial number density of all suprathermal species combined increases inward from the outer reaches of the magnetosphere, peaks between L=8-10, and then decreases inward. These L variations are qualitatively consistent with inward radial diffusion followed by rapid loss processes inside of L=8-10. Changes in composition as a function of L and energy are largely a result of differences in collision lifetimes with Saturn's neutral cloud.