Spectral Properties of He and Heavy Ions in ^3He-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.