Authors: B. Klecker, A.T. Bogdanov, M. Hilchenbach, A.T. Galvin, E. Moöbius, F.M. Ipavich, and P. Bochsler
Reference: COSPAR Colloquium ``The Outer Heliosphere: The Next Frontier''
Using data from the STOF experiment onboard SOHO we investigate the variation of suprathermal He+/He2+ abundances in the energy range 85-280 keV during the years 1997 to 1999. We observe a large variability of the He+ abundances ranging from He+/He2+ < 5% to ~ 1. The very large abundances are closely related with the passage of interplanetary shocks. Combining the data from STICS/WIND and STOF/SOHO we are able to identify in these events a pickup He+ distribution with the typical cutoff energy at twice the solar wind velocity and a suprathermal tail extending to a few 100 keV. We correlate daily averages of the He+ abundances of the suprathermal tail for all days with significant He+ flux with solar wind parameters and find a general anticorrelation of He+ abundances with solar wind velocity and solar thermal velocity. We discuss possible causes of this variability, in particular variations of the source strength of pickup ions and solar wind alphas and variations of the acceleration efficiency for He+ and He2+.