Generation of proton beams at switchback boundary-like rotational discontinuities in the solar wind

TL;DR

This study models Alfvénic rotational discontinuities in the solar wind to understand their role in trapping protons and generating beams, revealing the significance of electric fields and the potential for proton beam formation at switchback boundaries.

Contribution

It introduces a hybrid PIC simulation of boundary-like RDs, showing proton trapping and beam formation, and emphasizes the importance of electric fields and RD sub-structures in solar wind dynamics.

Findings

Protons can be trapped at boundary RDs, forming secondary beams.

Trapped protons exhibit high temperature anisotropy, exciting ion cyclotron waves.

Electric fields near RDs are crucial for proton trapping.

Abstract

Alfv\'enic rotational discontinuities (RDs) are abundant in the inner heliosphere and can be used to model the boundary of switchbacks, i.e. Alfv\'enic magnetic kinks. To investigate the effects of RDs on proton kinetics, we model a pair of switchback-boundary-like RDs with a hybrid Particle-In-Cell (PIC) approach in a 2D system. We find that, at one of the boundary RDs, a significant population of protons remains trapped over long times, creating a secondary beam-like component with temperature anisotropy $T_\perp/T_\|\gtrsim4$ in the proton velocity distribution function that excites ion cyclotron waves within the downstream portion of the transition layer. Further analysis suggests that the static electric field in the vicinity of the RD is the key factor in trapping the protons. This work indicates that switchback boundaries could represent a viable environment for the creation of proton beams in the heliosphere; it also highlights the need to investigate RD sub-structures, especially the embedded current systems of interplanetary RDs. Finally, this paper underscores the importance of high-resolution observations of the solar wind velocity distributions around RDs.