Wave coupling in partially ionized plasmas with shear flows

I. Fast-to-Alfvén transformation

¹ Departament de Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
² Institut d’Aplicacions Computacionals de Codi Comunitari (IAC3), Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain

^⋆ Corresponding author: miquel.cantallops@uib.cat

Received: 10 February 2025
Accepted: 1 August 2025

Abstract

We theoretically investigate the interplay between magnetohydrodynamic (MHD) waves and shear flows in a partially ionized solar plasma while focusing on the energy exchange mediated by the flow and the transformation between wave modes. We consider a simple model composed of a uniform partially ionized plasma with a straight magnetic field. A shear flow is present in the direction of the magnetic field with a velocity that varies linearly across the magnetic field. The linearized MHD equations in the single-fluid approximation are used, which include the ambipolar diffusion term due to ion-neutral collisions. A nonmodal approach was adopted in order to convert the flow spatial inhomogeneity into a temporal one, which added a temporal dependence into the component of the wave vector in the direction of the flow inhomogeneity. A system of three ordinary differential equations was derived, which generally governs the temporal evolution of the coupled MHD waves, their interaction with the shear flow, and their ambipolar damping. Numerical solutions were computed to study the coupling and mutual transformation between the fast magnetosonic wave and the Alfvén wave. A detailed parameter study was conducted demonstrating how the energy transfer and mode transformation are affected. The role of ambipolar diffusion was investigated by comparing the results of the ideal case with those obtained when ambipolar diffusion is included. We find that ambipolar diffusion can significantly affect the efficiency of the energy exchange between modes and that it introduces a new coupling mechanism. Additionally, a specific application to solar prominence threads is included, showing that wave coupling and energy exchange can occur within these and other similar structures in the solar atmosphere.