Parametric decays, stabilization effects, and electrostatic instabilities due to finite-amplitude Alfvén waves in a fast solar wind-like plasma

The effects of a finite‐amplitude Alfvén wave in a plasma system with a proton and an alpha‐particle beam, are studied. The system is described by a fourteenth‐order polynomial providing, thereby, many decay channels for the finite‐amplitude Alfvén wave. Apart from the parametric decays, the finite‐amplitude wave affects very drastically the stability properties of the linear system. It is shown that the large‐amplitude waves can either stabilize or destabilize further linear magnetosonic and ion‐cyclotron instabilities like in the case of one ion beam. However, as opposed to the case of one ion beam, for large enough amplitude of the finite‐amplitude wave, the effect on the linear system is always stabilizing. The finite‐amplitude waves can also trigger nonlinear purely electrostatic ion‐acoustic‐like instabilities. However, in the present case, even forward propagating Alfvén waves can destabilize electrostatic waves supported by the beams, something that does not happen when there is only one beam. Thus the saturation mechanism and the existence of nonlinear electrostatic waves in the presence of a finite‐amplitude wave is preserved in a two‐ion‐beam system. We believe that the present study reinforces previous results, involving only one ion‐beam, relative to unresolved issues in the fast solar wind.