Kinetic instabilities of multi-component plasma in the fast solar wind

Typical nonthermal features of ion velocity distributions observed in the fast solar wind are the relative streaming between two proton components and the anisotropic proton cores with T_{perpendicular to} > T_∥ , where the subscripts denote directions relative to the background magnetic field B₀. These nonthermal features lead to the growth of the several electromagnetic instabilities. Here linear Vlasov theory and one-dimensional hybrid simulations are used to study these in-stabilities in a homogeneous, magnetized and collisionless plasma model. Under these conditions, both magnetosonic and Alfvén/cyclotron modes become unstable. We show that for conditions typical of the fast solar wind the proton core temperature anisotropy plays a significant role in modifying the wave-particle scattering of each proton component as compared to the isotropic cases. Furthermore, this scattering reduces both the heating and anisotropy enhancement of the proton beam and decreases the relative proton/proton flow speed below the corresponding isotropic instability thresholds. This result provides additional support to the physical scenario in which instability thresholds correspond to observable constraints on plasma species anisotropies and match closer recent observation analysis.