Compressive high-frequency waves riding on an Alfv\'en/ion-cyclotron wave in a multi-fluid plasma

TL;DR

This study investigates high-frequency plasma waves superposed on large-amplitude Alfvén waves in a multi-fluid plasma, revealing mode couplings, instabilities, and potential implications for particle heating in space plasmas.

Contribution

Introduces a new eigenvalue method to analyze weakly-compressive high-frequency waves on Alfvén waves, accounting for inhomogeneity and mode coupling in a multi-fluid plasma.

Findings

Dispersion relations differ significantly from uniform plasma cases.

Compressibility excites acoustic waves and causes mode coupling.

Potential for wave-particle interactions leading to particle heating.

Abstract

In this paper, we study the weakly-compressive high-frequency plasma waves which are superposed on a large-amplitude Alfv\'en wave in a multi-fluid plasma consisting of protons, electrons, and alpha particles. For these waves, the plasma environment is inhomogenous due to the presence of the low-frequency Alfv\'en wave with a large amplitude, a situation that may apply to space plasmas such as the solar corona and solar wind. The dispersion relation of the plasma waves is determined from a linear stability analysis using a new eigenvalue method that is employed to solve the set of differential wave equations which describe the propagation of plasma waves along the direction of the constant component of the Alfv\'en wave magnetic field. This approach also allows one to consider weak compressive effects. In the presence of the background Alfv\'en wave, the dispersion branches obtained differ significantly from the situation of a uniform plasma. Due to compressibility, acoustic waves are excited and couplings between various modes occur, and even an instability of the compressive mode. In a kinetic treatment, these plasma waves would be natural candidates for Landau-resonant wave-particle interactions, and may thus via their damping lead to particle heating.