Electrostatic ion-acoustic-like instabilities in the solar wind with a backstreaming alpha particle beam

Nonlinear electrostatic instabilities have been shown to occur frequently and under very different conditions in a plasma with two ion beams such as the fast solar wind. These instabilities can be triggered when the phase velocity of electrostatic ion-acoustic propagating forward and backward relative to the interplanetary magnetic field overlap, due to the presence of a finite amplitude of circularly polarized wave. The instabilities can be triggered by waves supported by the same ion-component, or by waves supported by different ion-components. By assuming a beam of alpha particles moving backward relative to the external magnetic field, as observed in some events in the fast solar wind, it is shown that a very small negative drift velocity of the alpha particle beam relative to the core plasma - a few per cent of the local Alfvén velocity - can trigger a very rich variety of nonlinear electrostatic acoustic-like instabilities. Their growth rates can be rather large and they persist for a wide range of ion temperatures, provided that the alpha particle backstreaming beams, have much larger drift velocities.