Stochastic Acceleration of 3He and 4He in Solar Flares by Parallel-propagating Plasma Waves: General Results

We study the acceleration in solar flares of 3He and 4He from a thermal background by parallel-propagating plasma waves with a general broken power-law spectrum. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes, and the Coulomb collision loss and escape processes are included. Under the quasi-linear approximation, the pitch-angle-averaged acceleration time of α-particles is at least 1 order of magnitude longer than that of 3He ions at low energies and starts to approach that of 3He beyond a few tens of keV nucleon-1. Because their loss and escape times are comparable, the acceleration of 4He is suppressed significantly at low energies, and the spectrum of the accelerated α-particles is always softer than that of 3He. Quantitative results depend primarily on the wave generation and damping length scales, the electron plasma to gyrofrequency ratio, and the intensity of turbulence. The model gives a reasonable account of the observed low-energy 3He and 4He fluxes and spectra in the impulsive solar energetic particle events observed with the Advanced Composition Explorer. Other acceleration processes and/or stochastic acceleration by other wave modes seem to be required to explain the occasionally observed decrease of 3He to 4He ratio at energies beyond a few MeV nucleon-1.

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