Stochastic Acceleration of3He and4He by Parallel Propagating Plasma Waves

Stochastic acceleration of $^3$He and $^4$He from a thermal background byparallel propagating turbulent plasma waves with a single power-law spectrum ofthe wavenumber is studied. In the model, both ions interact with severalresonant modes. When one of these modes dominates, the acceleration rate isreduced considerably. At low energies, this happens for $^4$He, but not for$^3$He where contributions from the two stronger modes are comparable so thatacceleration of $^3$He is very efficient. As a result, the acceleration of$^4$He is suppressed by a barrier below $\sim 100$ keV nucleon$^{-1}$ and thereis a prominent quasi-thermal component in the $^4$He spectra, while almost allthe injected $^3$He ions are accelerated to high energies. This accounts forthe large enrichment of $^3$He at high energies observed in impulsive solarenergetic particle events. With reasonable plasma parameters this also providesa good fit to the spectra of both ions. Beyond $\sim 1$ MeV nucleon$^{-1}$, thespectrum of $^3$He is softer than that of $^4$He, which is consistent with theobserved decrease of the $^3$He to $^4$He ratio with energy. This study alsoindicates that the acceleration, Coulomb losses and diffusive escape of theparticles from the acceleration site {\it all} play important roles in shapingthe ion spectra. This can explain the varied spectral shapes observed recentlyby the {\it Advanced Composition Explorer}.Comment: 14 pages, 4 figures, Submitted to Apj