Alpha Core-Beam Origin in Low-$\beta$ Solar Wind Plasma: Insights from Fully Kinetic Simulation

TL;DR

This paper uses fully kinetic simulations to demonstrate that non-linear Landau damping can generate alpha-particle beams in low-beta solar wind plasma, explaining observed ion drift velocities.

Contribution

It introduces a novel mechanism for alpha-beam formation through non-linear Landau damping validated by kinetic simulations and linear theory.

Findings

Alpha particles form beams via non-linear Landau damping.

Simulation results match linear theory predictions.

The mechanism explains ion drift constraints in solar wind.

Abstract

In-situ observations of the fast solar wind in the inner-heliosphere show that minor ions and ion sub-populations often exhibit distinct drift velocities. Both alpha particles and proton beams stream at speeds that rarely exceed the local Alfv\'{e}n speed relative to the core protons, suggesting the presence of instabilities that constrain their maximum drift. We aim to propose a mechanism that generates an alpha-particle beam through non-linear Landau damping, primarily driven by the relative super-Alfv\'{e}nic drift between protons and alpha particles. To investigate this process, we perform one-dimensional, fully kinetic particle-in-cell simulations of a non-equilibrium multi-species plasma, complemented by its linear theory to validate the model during the linear phase. Our results provide clear evidence that the system evolves by producing an alpha-particle beam, thereby suggesting a local mechanism for alpha-beam generation via non-linear Landau damping.