Proton heating by pick-up ion driven cyclotron waves in the outer heliosphere: Hybrid expanding box simulations

TL;DR

This study uses hybrid expanding box simulations to explore how pick-up ion driven cyclotron waves heat solar wind protons in the outer heliosphere, revealing efficient perpendicular and parallel heating mechanisms.

Contribution

It introduces a hybrid simulation approach to model proton heating by cyclotron waves driven by pick-up ions, accounting for expansion and charge-exchange effects.

Findings

Pick-up protons form unstable ring distributions generating Alfvén cyclotron waves.

Alfvén cyclotron waves cause perpendicular proton heating via cyclotron resonance.

Ion acoustic waves heat protons in the parallel direction through Landau resonance.

Abstract

Using one-dimensional hybrid expanding box model we investigate properties of the solar wind in the outer heliosphere. We assume a proton-electron plasma with a strictly transverse ambient magnetic field and, beside the expansion, we take into account influence of a continuous injection of cold pick-up protons through the charge-exchange process between the solar wind protons and hydrogen of interstellar origin. The injected cold pick-up protons form a ring distribution function that rapidly becomes unstable and generate Alfv\'en cyclotron waves. The Alfv\'en cyclotron waves scatter pick-up protons to a spherical shell distribution function that thickens over that time owing to the expansion-driven cooling. The Alf\'ven cyclotron waves heat solar wind protons in the perpendicular direction (with respect to the ambient magnetic field) through the cyclotron resonance. At later times, the Alfv\'en cyclotron waves become parametrically unstable and the generated ion acoustic waves heat protons in the parallel direction through the Landau resonance. The resulting heating of the solar wind protons is efficient on the expansion time scale.