On the variability of suprathermal He+ ions at 1 AU
Abstract
Using data from the STOF sensor of the CELIAS experiment onboard SOHO we investigate the variation of suprathermal He+ /He2+ abundance ratios in the energy range 85-280 keV during the years 1997 to 1999. We observe a large variability of the He+ abundances in solar energetic particle events ranging from He+ /He2+ <5% to He+ /He2+ > 1. Large He+ abundances are closely related with the passage of interplanetary shocks, whereas abundance ratios of ~ 0.15 have been observed at 1 AU during CIR events. Combining the data from STICS/WIND and SOHO/ CELIAS/STOF for one of the events with He+ /He2+ ~ 1, a pickup He+ distribution with the typical cutoff energy at twice the solar wind velocity and a suprathermal tail extending to ~ 150 keV/nuc is observed. We correlate 12 hour averages of the He+ abundances in the suprathermal tail for all days with significant He flux with solar wind parameters and find a general anti-correlation of He+ abundances with the solar wind velocity and the solar wind thermal velocity, with a large scatter superimposed on the general trend. We discuss possible causes of this variability, in particular variations of the solar wind and pickup ion source strength.
- Publication:
-
International Cosmic Ray Conference
- Pub Date:
- August 2001
- Bibcode:
- 2001ICRC....8.3100K