Relative Abundance Variations of Energetic He⁺/He²⁺ in CME Related SEP Events Available to Purchase

We have investigated several CME‐related SEP events with unusually high abundance of He⁺ relative to He²⁺ 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 plasma in CME’s. We have investigated in detail the temporal evolution and the energy spectra of these events. We find that the maximum of the He⁺/He²⁺ ratio usually coincides with the arrival of the shock or a solar wind structure. This is a strong indication for local acceleration of these ions. The He⁺ enhancement does not seem to be associated with cold plasma within CME’s itself. 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. Also, the observed temporal variability decreases with increasing energy. These two results seem to indicate two different populations for He⁺ and He²⁺ with different energy spectra.