Spectral Properties of He and Heavy Ions in 3 He‐rich Solar Flares

Abstract

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 nucleon -1 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 nucleon -1. In most of the events there was even stronger hardening of the 3He spectrum below ∼1 MeV nucleon-1, 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.