3He enrichments by magnetosonic waves in current-driven instabilities

Abstract

A theoretical model for3He enrichments in solar energetic particles is developed. First, current-driven, electrostatic instabilities that have frequencies $$\omega \approx n\Omega 3_{{\rm H}e} $$ ( $$\Omega 3_{{\rm H}e} $$ is the cyclotron frequency of3He) are investigated for a plasma consisting of H,4He,3He, and electrons with the density of3He much lower than those of H and4He. It is found that in many cases the oblique ion-acoustic waves can have positive growth rates at frequencies $$\omega \approx \Omega 3_{{\rm H}e} $$ and, at the same time, negative growth rates at $$\omega \approx \Omega 4_{{\rm H}e} $$ and atω ≈ ΩH. This can occur near the marginal state of the instability. The wave damping at these frequencies is caused by the cyclotron resonances of4He and H. The cyclotron damping at $$\omega \approx \Omega 3_{{\rm H}e} $$ is negligible, however, because the abundance of3He is very small. The H cyclotron waves can be unstable at $$\omega \approx 2\Omega 3_{{\rm H}e} $$ for a wide region of plasma parameters; the electron-to-ion temperature ratio must beT e /T H ≳ 1.5. To destabilize the4He cyclotron waves with $$\omega \approx \Omega 3_{{\rm H}e} $$ , high4He density and high electron temperature are both required. Then,3He enrichments are studied on the basis of the theory of nonlinear magnetosonic waves, which can promptly accelerate ions. The current-driven electrostatic waves with $$\omega \approx n\Omega 3_{{\rm H}e} $$ can enhance fluctuation velocities of3He. Thus, in the presence of these waves, magnetosonic waves can selectively accelerate3He particles to high energies. Finally, cyclotron resonances of heavy ions with the waves $$\omega \approx n\Omega 3_{{\rm H}e} $$ or $$\omega< \Omega 4_{{\rm H}e} $$ are briefly discussed.