Particle acceleration by counter-propagating circularly polarized Alfv\'en waves

TL;DR

This paper investigates how counter-propagating circularly polarized Alfvén waves can efficiently accelerate particles to relativistic energies, with potential applications in astrophysics and laboratory experiments.

Contribution

It demonstrates a novel particle acceleration mechanism driven by counter-propagating Alfvén waves and identifies critical wave amplitude thresholds for irreversible relativistic acceleration.

Findings

Particles gain relativistic energy rapidly once wave amplitudes exceed critical values.

Higher wave phase velocity reduces the critical amplitude needed for acceleration.

Maximum particle energy increases with lower wavenumber and frequency.

Abstract

Counterpropagating Alfv\'en waves are ubiquitously observed in many astrophysical environments, such as a star surface and a planetary foreshock. We discuss an efficient particle acceleration mechanism in two counterpropagating circularly polarized Alfv\'en waves. Phase transitions of particle behavior occur when wave amplitudes exceed two critical values. Above the critical amplitudes, the numerical simulation shows that any particles irreversibly gain relativistic energy within a short time regardless of their initial position and energy once the coherent waveform is formed. The accelerated particles have spatial coherence. Higher wave phase velocity requires smaller critical amplitudes, while the maximum attainable energy increases as the wavenumber and the frequency decrease. The results may be applicable in some astrophysical phenomena, as well as a future laboratory experiment using high-power lasers.