No abstract

In a recent paper, Hollweg et al. (1993) studied the parametric decay of Alfvén waves in high‐speed solar wind streams. Following this analysis, we consider the nonlinear decay of left‐hand‐polarized ion cyclotron waves. It is shown that in a solar wind type plasma composed of electrons, protons, and alpha particles drifting relative to the protons, both branches of the dispersion relation of the circularly polarized waves can be excited by observed thermal anisotropies (Gomberoff and Elgueta, 1991). Guided by this analysis, the parametric decay of each branch of the dispersion relation is discussed. It is shown that the presence of drifting alpha particles introduces new wave couplings in the system that lead to new instabilities. Some of these instabilities involve sound waves supported essentially by the alpha particles, which, due to Landau damping, can be very efficient in the energization of alpha particles. Other instabilities involve ordinary sound waves that can lead to proton heating. A modulational instability that involves two electromagnetic daughters is also found. We have also found that a strong pump can force decays of modes that do not satisfy the resonance conditions when the pump intensity is vanishingly small. Finally, it is shown that both branches of the dispersion relation, particularly the branch close to the Doppler‐shifted alpha particle resonance, are highly unstable even for small intensities of the pump wave.

We study the dispersion relation of left‐hand‐polarized Alfvén waves in multicomponent plasmas. If initially the plasma components are not drifting relative to each other, the Alfvén waves propagate until they meet the gyrofrequency of the species with the largest Ml = ml/zlmp value (ml is the ion mass, zl is the degree of ionization, and mp is the proton mass). As a result of resonance absorption, these ions are heated and accelerated by quasi‐linear resonant interaction. When these ions reach a given velocity, the dispersion relation of the Alfvén waves changes drastically. The Alfvén branch of the dispersion relations no longer goes to the gyrofrequency of the ion species with the largest Ml value, but it goes to the gyrofrequency of the species with the second largest Ml value. In this way, more and more channels open up. We apply these results to high‐speed solar wind streams, and we argue that Alfvén waves generated in coronal holes or in nearby regions can heat and accelerate heavy ions.