Nonlinear electrostatic instabilities in a multi-ion plasma with two drifting ion- species

It is shown that the presence of a finite amplitude Alfvén-cyclotron in a solar wind-like plasma with two drifting ion-components relative to the background plasma, leads to electrostatic ion-acoustic-like waves triggered by the interaction of a forward propagating linear ion-acoustic wave supported by one species, and an ion-acoustic backward propagating ion-acoustic waves supported by a different ion-species. This is shown to occur in fast solar wind streams with drifting protons and alpha-particles. Linear ion-acoustic instabilities of this kind are known to occur when the phase velocity of an ion-acoustic wave moving forward and supported by one species, overlaps with the phase velocity of a backward propagating ion-acoustic wave supported by another ion-species. However, not only their properties are completely different, but also, while the latter are linear instabilities -they do not require the presence of a large amplitude Alfvén-cyclotron wave- the former are nonlinear instabilities and require at least two different drifting ion species relative to the core protons. It has been recently shown that the electrostatic turbulence in space plasmas consists of longitudinal waves with acoustic-like dispersion relation [Valentini et al., Phys. Rev. Lett. 100, 125003, 2008], so that these type of waves might also play an important role in several ion-component plasmas such as the solar wind and the magnetosphere.