Relative drifts and temperature anisotropies of protons and α particles in the expanding solar wind: 2.5D hybrid simulations

Context. We perform 2.5D hybrid simulations to investigate the origin and evolution of relative drift speeds between protons and a particles in the collisionless turbulent low-(beta) over tilde $ solar wind plasma. Aims. We study the generation of differential streaming by wave-particle interactions and absorption of turbulent wave spectra. Next we focus on the role of the relative drifts for the turbulent heating and acceleration of ions in the collisionless fast solar wind streams. Methods. The energy source is given by an initial broad-band spectrum of parallel propagating Alfven-cyclotron waves, which coexists with the plasma and is self-consistently coupled to the perpendicular ion bulk velocities. We include the effect of a gradual solar wind expansion, which cools and decelerates the minor ions. We here consider for the first time the combined effect of self-consistently initialized dispersive turbulent Alfvenic spectra with differentially streaming protons and alpha particles in the expanding solar wind outflows within a 2.5D hybrid simulation study. Results. For differential streaming of V-ap < 0.5V(A), the selected initial wave spectrum accelerates the minor ions in the non-expanding wind. At V-ap = 0.5V(A) the relative drift speed remains nearly steady. For ions that stream below this threshold value, the waves act to increase the magnitude of the relative drift speed. Ions that stream faster than the threshold value become subject to a nonlinear streaming instability, and as the system evolves, their bulk velocities decrease. We find that the solar wind expansion strongly affects the relative drift speed and significantly slows down both ion species for all values of the relative drift speeds considered in this study. The initial nonresonant wave spectra interact with the particles, resulting in preferential and anisotropic heating for the minor ions with a prominent increase of their perpendicular temperature, which overcomes the effect of the double-adiabatic cooling that is due to the solar wind expansion. Finally, the initial parallel spectra undergo a micro-turbulent nonlinear cascade during which oblique waves are generated, whose intensity depends on the value of the relative drift speed.