On nonlinear Alfv\'en-cyclotron waves in multi-species plasma

TL;DR

This paper investigates the nonlinear interaction between Alfvén, cyclotron, and acoustic waves in multi-species plasmas, revealing a mechanism for wave energy cascade and potential implications for solar wind turbulence and particle heating.

Contribution

It introduces a new set of coupled wave equations for multi-species plasmas, elucidating the nonlinear coupling of Alfvén waves to acoustic and cyclotron waves.

Findings

Mutually coupled Alfvén-cyclotron-acoustic waves can be excited in multi-species plasmas.

The nonlinear process can drive a cascade of wave energy and generate microturbulence.

Electric fluctuations may influence turbulence dissipation and particle heating in the solar wind.

Abstract

Large-amplitude Alfv\'en waves are ubiquitous in space plasmas and a main component of magnetohydrodynamic (MHD) turbulence in the heliosphere. As pump waves they are prone to parametric instability by which they can generate cyclotron and acoustic daughter waves. Here we revisit a related process within the framework of the multi-fluid equations for a plasma consisting of many species. The nonlinear coupling of the Alfv\'en wave to acoustic waves is studied, and a set of compressive and coupled wave equations for the transverse magnetic field and longitudinal electric field is derived for waves propagating along the mean-field direction. It turns out that slightly compressive Alfv\'en waves exert, through induced gyro-radius and kinetic-energy modulations, an electromotive force on the particles in association with a longitudinal electric field, which has a potential that is given by the gradient of the transverse kinetic energy of the particles gyrating about the mean field. This in turn drives electric fluctuations (sound and ion-acoustic waves) along the mean magnetic field, which can nonlinearly react back on the transverse magnetic field. Mutually coupled Alfv\'en-cyclotron-acoustic waves are thus excited, a nonlinear process that can drive a cascade of wave energy in the plasma and may generate compressive microturbulence. These driven electric fluctuations might have consequences for the dissipation of MHD turbulence and, thus, for the heating and acceleration of particles in the solar wind.