Spectral Properties of He and Heavy Ions in³He‐rich Solar Flares

Using advanced instrumentation on the ACE spacecraft, we have conducted a survey of solar energetic particle spectra in 3 He-rich events over a broad energy range $80 keV nucleon 1 to 15 MeV nucleon 1 during the period 1997 September-2001 March. The spectra of 4 He and heavy ions (C, N, O, Ne, Mg, Si, S, Ca, Fe) were generally similar over this range but often hardened below $1 MeV nucleon 1 . In most of the events there was even stronger hardening of the 3 He spectrum below $1 MeV nucleon 1 , leading to an energydependent 3 He : 4 He ratio. These observations point to unique and distinct properties of 3 He in these events and place new constraints on models that seek to explain enhancements of 3 He and heavy ions using the same mechanisms. In addition to the events with spectra in the form of power laws or double power laws, there is a second class of event in which the low-energy 3 He and Fe spectra are rounded, while the 4 He remains a power law. In these cases 3 He and Fe spectra can be fitted at low energies by a stochastic acceleration model, but this model does not explain the higher energy portions of these spectra, nor the power-law spectral forms of the 4 He. These observations appear to require an additional mechanism, such as acceleration by cascading MHD turbulence. The 3 He enrichment pattern that we observe suggests that all these different spectral features might be due to processes with a common origin but then followed by different acceleration histories.