Comparisons of the Suprathermal He+ Spectrum in Saturn's Magnetosphere with the Pickup He+ Spectrum Upstream of Saturn's Bow Shock

Occasional large decreases in the flux of suprathermal He+ and He++ have been observed in Saturn’s magnetosphere (Hamilton et al, SM14A-06, Spring 2009 AGU meeting). These decreases last for a few days and are indicative of dynamic changes in the entry of solar wind material into the magnetosphere and possibly also its energization. In this paper we use data from the Charge-Energy-Mass Spectrometer (CHEMS) on Cassini to compare the long term average spectrum of He+ (3-220 keV) in the Saturn’s outer magnetosphere (16-21 Rs) with a similar long term average in interplanetary space from the first half of 2004 during Cassini’s approach to Saturn. He+ and He++ are unique tracer species since they have no local sources within Saturn’s magnetosphere. He++ forms a portion of the core solar wind while He+ exists as a suprathermal pick-up ion population carried by the solar wind. We find that the interplanetary flux of He+ exceeds that in Saturn’s outer magnetosphere by a factor of two to three up to about 25 keV while above that energy the fluxes are nearly equal, with the same power law spectral slope. We note that the long-term interplanetary average includes both quite periods and shock-associated acceleration events. In contrast the interplanetary H+ spectrum is much lower (factor of 5-100, depending on energy) than that observed in the outer magnetosphere, indicative of strong local sources of H+. The interplanetary He++ spectrum is also lower than the outer magnetospheric spectrum by about a factor of 10 above 16 keV/e, where it can be measured within the magnetosphere. We will compare these long term averages with the shorter periods of depressed He+ and He++ flux observed previously.