Relative drifts and temperature anisotropies of protons and $\alpha$ particles in the expanding solar wind -- 2.5D hybrid simulations

TL;DR

This study uses 2.5D hybrid simulations to explore how wave-particle interactions and solar wind expansion influence the relative drift speeds and heating of protons and alpha particles in the collisionless solar wind.

Contribution

It is the first to self-consistently combine dispersive turbulent Alfvénic spectra with differential ion streaming in an expanding solar wind within a hybrid simulation framework.

Findings

A threshold drift speed of 0.5 V_A exists where relative drift remains steady.

Waves increase drift speeds below the threshold, but cause deceleration above it.

Solar wind expansion significantly reduces ion drift speeds across all conditions.

Abstract

We perform 2.5D hybrid simulations to investigate the origin and evolution of relative drift speeds between protons and $\alpha$ particles in the collisionless turbulent low-$\beta$ solar wind plasma. 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. The energy source is given by an initial broad-band spectrum of parallel propagating Alfv\'en-cyclotron waves, which co-exists 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. This paper for the first time considers the combined effect of self-consistently initialized dispersive turbulent Alfv\'enic spectra with differentially streaming protons and $\alpha$ particles in the expanding solar wind outflows within a 2.5D hybrid simulation study. In the non-expanding wind, we find a threshold value of the differential streaming $V_{\alpha p} = 0.5 V_\mathrm{A}$, for which the relative drift speed remains nearly steady. For ions, streaming below the threshold value, the waves act to increase the magnitude of the relative drift speed. Ions, which stream faster than the threshold value become subject to nonlinear streaming instability and as the system evolves their bulk velocities decrease. We find that the solar wind expansion strongly affects the relative drift speeds and significantly slows down both ion species for all values of the relative drift speeds considered in this study.