Long term time variations of the suprathermal ions in Saturn's magnetosphere

Abstract

[1] We use the Charge-Energy-Mass Spectrometer (CHEMS) on Cassini to study long-term time variations of the suprathermal (E/Q = 27–220 keV/e) ions in Saturn's equatorial ring current (Latitude = −10° to 10°, dipole L = 7–16) from the end of 2004 to the end of 2010. We identify five equatorial time periods, which vary in length from 124 to 282 days, and determine the average ion composition of the ring current for each period. The species examined are the water group W+ (O+, OH+, H2O+, and H3O+), H+, H2+, He++, He+, and O++. We find that the combined partial number density over this energy range varies less than a factor of two from a minimum of 1.3 × 10−3 cm−3 to a maximum of 2.3 × 10−3 cm−3, implying that long-term time variations in neutral source strength must be modest. The most abundant species (63% W+, 30% H+, 5% H2+) show no long-term trends. The H+/W+ and H2+/W+ ratios vary less than half as much as individual densities. The smaller variations in the ratios could be understood if all three species mostly originate from the Enceladus plumes either directly or via dissociation, but our results can accommodate differing neutral sources (currently thought to be Enceladus for W, Saturn's atmosphere for H, and Titan for H2) if the suprathermal ion density variations are largely caused by changes in the overall acceleration rate of the thermal plasma. Both He++, originating from the solar wind, and He+, interplanetary pickup ions of interstellar origin, show a substantial decrease in 2009–2010 near solar minimum.