Multi-fluid approach to high-frequency waves in plasmas: I. Small-amplitude regime in fully ionized medium

TL;DR

This study investigates high-frequency wave behavior in fully ionized plasmas using a multi-fluid model, revealing ion decoupling, damping effects, and the impact of collisions on wave propagation relevant to solar plasma modeling.

Contribution

It introduces a multi-fluid approach including collisions and Hall effects to better describe high-frequency plasma waves beyond ideal MHD assumptions.

Findings

High-frequency ions are less coupled than low-frequency ions.

Collisions cause significant damping of high-frequency waves.

Coulomb collisions eliminate cyclotron resonances and cut-off regions.

Abstract

Ideal MHD provides an accurate description of low-frequency Alfv\'en waves in fully ionized plasmas. However, higher frequency waves in many plasmas of the solar atmosphere cannot be correctly described by ideal MHD and a more accurate model is required. Here, we study the properties of small-amplitude incompressible perturbations in both the low and the high frequency ranges in plasmas composed of several ionized species. We use a multi-fluid approach and take into account the effects of collisions between ions and the inclusion of Hall's term in the induction equation. Through the analysis of the corresponding dispersion relations and numerical simulations we check that at high frequencies ions of different species are not as strongly coupled as in the low frequency limit. Hence, they cannot be treated as a single fluid. In addition, elastic collisions between the distinct ionized species are not negligible for high frequency waves since an appreciable damping is obtained. Furthermore, Coulomb collisions between ions remove the cyclotron resonances and the strict cut-off regions that are present when collisions are not taken into account. The implications of these results for the modelling of high-frequency waves in solar plasmas are discussed.