Effect of Alpha Beams on Low-frequency Electromagnetic Waves Driven by Proton Beams

Abstract

Abstract Electromagnetic waves (EMWs) below or near the proton gyrofrequency can be left-hand (LH) or right-hand (RH) polarized waves, which are believed to be fundamentally important in the energization of plasma particles. Proton and alpha beams that are associated with EMW activities are ubiquitous in space and astrophysical plasmas. Based upon linear Vlasov theory, we study the effect of alpha beams on the LH and RH instabilities driven by both the presence of proton and alpha beam populations in a compensated-current system. The results show that the thresholds, real frequencies, and growth rates of both instabilities are highly sensitive to the density and drift velocity of alpha beams. In particular, alpha beams with inhibit two kinds of instabilities; where is the drift velocity of alpha beams with minimum values of growth rates, while for both the growth rates are enhanced with the density or drift velocity of alpha beams, especially for the LH waves. We also investigate the competition between the LH and RH instabilities. The RH waves have a lower threshold and higher growth rate than the LH waves. Additionally, a comparison of the approximate analytical solutions with the exact numerical calculations based on WHAMP indicates that the analytical results are in good agreement with the numerical calculations. A possible application to EMW activities with respect to the formation and evolution of ion beams in the solar wind is briefly discussed.