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The Astrophysical Journal


Using advanced instrumentation on the ACE spacecraft, we have conducted a survey of solar energetic particle spectra in ³He-rich events over a broad energy range ∼80 keV nucleon-1 to 15 MeV nucleonˉ¹ during the period 1997 September-2001 March. The spectra of ⁴He and heavy ions (C, N, O, Ne, Mg, Si, S, Ca, Fe) were generally similar over this range but often hardened below ∼1 MeV nucleonˉ¹. In most of the events there was even stronger hardening of the ³He spectrum below ∼1 MeV nucleonˉ¹, leading to an energy-dependent ³He: ⁴He ratio. These observations point to unique and distinct properties of ³He in these events and place new constraints on models that seek to explain enhancements of ³He 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 ³He and Fe spectra are rounded, while the ⁴He remains a power law. In these cases ³He 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 ⁴He. These observations appear to require an additional mechanism, such as acceleration by cascading MHD turbulence. The ³He 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.



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