Stochastic Acceleration of 3 He and 4 He in Solar Flares by Parallel Propagating Plasma Waves: General Results
Abstract
We study the acceleration in solar flares of
3
He and
4
He from a thermal background by parallel propagating plasma waves with a general broken power-law spectrum that takes into account the turbulence generation processes at large scales and the thermal damping effects at small scales. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes. Because low-energy
α
-particles only interact with small scale waves in the
4
He-cyclotron branch, where the wave frequencies are below the
α
-particle gyro-frequency, their pitch angle averaged acceleration time is at least one order of magnitude longer than that of
3
He ions, which mostly resonate with relatively higher frequency waves in the proton-cyclotron (PC) branch. The
α
-particle acceleration rate starts to approach that of
3
He beyond a few tens of keV nucleon
−1
, where
α
-particles can also interact with long wavelength waves in the PC branch. However, the
4
He acceleration rate is always smaller than that of
3
He. Consequently, the acceleration of
4
He is suppressed significantly at low energies, and the spectrum of the accelerated
α
-particles is always softer than that of
3
He. The model gives reasonable account of the observed low-energy
3
He and
4
He fluxes and spectra in the impulsive solar energetic particle events observed with the {\it Advanced Composition Explorer}. We explore the model parameter space to show how observations may be used to constrain the model.