ar X iv : a st ro - p h / 04 03 00 7 v 1 2 8 Fe b 20 04 Stochastic Acceleration of 3 He and 4 He by Parallel Propagating Plasma Waves

Stochastic acceleration of He and He 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 He, but not for He where contributions from the two stronger modes are comparable so that acceleration of He is very efficient. As a result, the acceleration of He is suppressed by a barrier below ∼ 100 keV nucleon and there is a prominent quasi-thermal component in the He spectra, while almost all the injected He ions are accelerated to high energies. This accounts for the large enrichment of He 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, the spectrum of He is softer than that of He, which is consistent with the observed decrease of the He to He 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. Subject headings: acceleration of particles — plasma — Sun: abundances — Sun: flares — turbulence Center for Space Science and Astrophysics, Department of Physics, Stanford University, Stanford, CA 94305; [email protected] Department of Physics and Applied Physics, Stanford University, Stanford, CA 94305; [email protected] Department of Physics and Institute for Physical Sciences and Technology, University of Maryland, College Park, MD 20742-4111; [email protected]