Fast-magnetosonic/whistler wave instabilities in multi-ion solar wind plasmas: effects of alpha particles and temperature anisotropy

Abstract

Differential flows among ion species and ion temperature anisotropies are commonly observed in the solar wind, providing free energy to drive several types of wave instabilities. Although the instability of fast-magnetosonic/whistler (FM/W) waves has been studied extensively, a comprehensive analytical investigation using kinetic theory remains lacking. In this study, we analytically investigate the instability of right-hand circularly polarized FM/W waves propagating parallel to the ambient magnetic field in a collisionless, multi-ion solar wind plasma, focusing on the roles of proton and alpha particle beams and temperature anisotropy. Employing kinetic theory and Lorentz transformations, we derive expressions for the real frequency and growth rate of this instability and analyse the influence of key plasma parameters such as drift velocity, anisotropy, and relative ion density. These results confirm that alpha beams can drive the FM/W instability more efficiently than proton beams under similar conditions, owing to their distinct charge-to-mass ratio and thermal properties, particularly under super-Alfvénic drift conditions. In particular, we focus on the effects of ion temperature anisotropy and drifting alpha particles. Our results indicate that alpha particle anisotropy significantly alters the instability threshold and growth rate: reducing anisotropy enhances the instability by increasing the growth rate and lowering the beam velocity threshold. To validate our analytical findings, theoretical instability thresholds are validated against Wind spacecraft observations at 1 au, showing good agreement and confirming the FM/W instability’s role in limiting ion drift and anisotropy in the solar wind. This study advances the understanding of kinetic mechanisms driving FM/W instabilities in multi-ion solar wind plasmas.