Electrostatic instabilities triggered by finite amplitude circularly polarized waves

Finite amplitude circularly polarized waves are known to be able to change the properties of linear proton beam-plasma instabilities (Gomberoff, 2003a; Gomberoff et al., 2003b). Electrostatic waves can be destabilized by the presence of large amplitude left-hand polarized waves (Gomberoff et al., 2004a,b). Here, it is shown that electrostatic waves can also be destabilized by finite amplitude waves, propagating backward relative to the external magnetic field. These instabilities occur when the phase velocities of the forward and backward propagating ion-acoustic waves relative to the proton core or to the proton beam, become equal.