On the variability of suprathermal He+ ions at 1 AU

Details On the variability of suprathermal He+ ions at 1 AU On the variability of suprathermal He+ ions at 1 AU

Aug 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icrc....8.3100k&link_type=abstract

Proceedings of the 27th International Cosmic Ray Conference. 07-15 August, 2001. Hamburg, Germany. Under the auspices of the Int

Physics

Scientific paper

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.

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