Abundance Variations of Energetic He+ in CME Related SEP Events

Physics

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2111 Ejecta, Driver Gases, And Magnetic Clouds, 2139 Interplanetary Shocks, 2152 Pickup Ions, 2169 Sources Of The Solar Wind

Scientific paper

We have investigated several CME related SEP events with unusually high abundance in He+ in the energetic particle population which have been observed between 1998 and 2000 with ACE SEPICA and SOHO CELIAS. Usually the abundance of He+ is below a few percent whereas at these times the He+/He2+ ratio can be closer to one. Possible sources for He+ are interstellar pickup ions or cold solar ejecta in CMEs. The temporal evolution and the energy spectra of these events have been investigated in detail. The maximum of the He+/He2+ ratio usually coincides with the arrival of the shock or a discontinuity. This seems to suggest local acceleration of these ions. The He+ enhancement does not seem to be associated with cold ejecta in the CME. Therefore, most probably interstellar pickup ions are the source for the He+ enhancement. Furthermore, the He+/He2+ ratio appears to be consistently lower at higher energies, and the observed temporal variability decreases with increasing energy. This seems to indicate two different populations for He+ and He2+ with different energy spectra.

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