Spectral properties of He and heavy ions in 3He-rich solar flares

Using advanced instrumentation on the ACE spacecraft, we have conducted a survey of solar energetic particle spectra in He-3-rich events over a broad energy range similar to80 keV nucleon(-1) to 15 MeV nucleon(-1) during the period 1997 September 2001 March. The spectra of He-4 and heavy ions ( C, N, O, Ne, Mg, Si, S, Ca, Fe) were generally similar over this range but often hardened below similar to1 MeV nucleon(-1). In most of the events there was even stronger hardening of the He-3 spectrum below similar to1 MeV nucleon(-1), leading to an energy-dependent He-3 : He-4 ratio. These observations point to unique and distinct properties of He-3 in these events and place new constraints on models that seek to explain enhancements of He-3 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 He-3 and Fe spectra are rounded, while the He-4 remains a power law. In these cases He-3 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 He-4. These observations appear to require an additional mechanism, such as acceleration by cascading MHD turbulence. The He-3 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.