Stochastic Acceleration of 3He and 4He by Parallel Propagating Plasma Waves
Abstract
Stochastic acceleration of 3He and 4He from a thermal background by parallel propagating turbulent plasma waves with a single power-law spectrum of the wavenumber is studied. In the model, both ions interact with several resonant modes. When one of these modes dominates, the acceleration rate is reduced considerably. At low energies, this happens for 4He, but not for 3He where contributions from the two stronger modes are comparable so that acceleration of 3He is very efficient. As a result, the acceleration of 4He is suppressed by a barrier below 100 keV nucleon-1 and there is a prominent quasi-thermal component in the 4He spectra, while almost all the injected 3He ions are accelerated to high energies. This accounts for the large enrichment of 3He at high energies observed in impulsive solar energetic particle events. With reasonable plasma parameters this also provides a good fit to the spectra of both ions. Beyond 1 MeV nucleon-1, the spectrum of 3He is softer than that of 4He, which is consistent with the observed decrease of the 3He to 4He ratio with energy. This study also indicates that the acceleration, Coulomb losses and diffusive escape of the particles from the acceleration site all play important roles in shaping the ion spectra. This can explain the varied spectral shapes observed recently by the Advanced Composition Explorer.
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