A study of Alfvén Wave and Proton Beam Interactions with Expanding Box Hybrid Simulations

Field aligned proton beams drifting with respect to the core proton population, slightly exceeding the local Alfvén speed, are commonly observed in the Alfvénic solar wind. Understanding the origin, evolution, and stability of proton beams in the expanding solar wind, and how they interact with waves and fields, is fundamental in understanding solar wind dynamics and heating. In this work, wave-particle interactions mediated by the collapse of an Alfvén wave are investigated by means of the hybrid expanding box model, a code that mimics dynamical effects introduced by the solar wind radial expansion within the hybrid-PIC framework. Starting with an amplitude-modulated Alfvén wave, I will show that the Alfvén wave undergoes a local collapse leading to the formation of a field-aligned beam drifting at the Alfvén speed. I will discuss the initial wave collapse and how the radial expansion of the wind affects onset of kinetic instabilities and wave-particle interactions over long time scales, by considering an ensemble of initial conditions matching solar wind observations at a radial distance of about 0.3 AU. I will compare simulation results with solar wind particle and magnetic field data at various radial distances and discuss the implications of this work to interpret solar wind observations.