The Variability of Suprathermal Pickup He+ Measured with SOHO/CELIAS/STOF

Abstract:

SOHO/CELIAS/STOF measures suprathermal ions which are preferentially accelerated and end up as energetic particles. We have investigated temporal variations of He+ which has its origin in interstellar and inner-source pickup ions but could also - occasionally - be solar. We found no interstellar pickup He+ during solar quiet times in the energy-per-charge range from 35 to 660 keV/e. This gives an upper limit to the efficiency of the Gloeckler-Fisk v^-5 mechanism. Using a combination of SOHO/CELIAS/PM and ACE/MAG data we investigate the He+ abundances in Co-rotating Interaction Regions (CIRs) during the years 1997 to 2007. We find that suprathermal pickup He+ mainly appears inside the compressed and decelerated fast solar wind region of (CIRs) and persists within the leading parts of the fast wind. This is also found in CIRs which have no associated forward or reverse shocks, the majority of CIRs at 1 AU. This observation could be explained by an injection and acceleration process due to a statistical mechanism in the compressed solar wind such as the Gloeckler-Fisk v^-5 mechanism. In addition, we find an increase in the He+/He++ ratio in December which we believe to be a signature of SOHO crossing the focusing cone of interstellar neutral He. This analysis is based on a re-evaluation and careful modeling of the substantial background which is present in SOHO/CELIAS/STOF. Using data from SOHO/COSTEP/EPHIN, SOHO/CELIAS/STOF, and SOHO/CELIAS/SEM, we show that UV is not suppressed efficiently by the collimator of STOF and triggers the Time-of-Flight unit by generating photoelectrons. The second and third triggers are then provided by penetrating energetic particles which hit STOF's silicon solid-state detectors.