Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach

TL;DR

This paper derives and compares two-fluid and single-fluid models of magnetohydrodynamic waves in partially ionized solar plasmas, revealing significant differences at higher frequencies and introducing a new slow wave mode.

Contribution

It provides a detailed two-fluid analysis of MHD waves in partially ionized plasmas, highlighting the limitations of single-fluid models at high frequencies and identifying a new slow wave mode.

Findings

Two- and single-fluid models agree at low frequencies.

Maximum damping occurs at specific frequencies related to ion-neutral collision frequency.

A new slow magneto-acoustic wave mode appears at higher frequencies.

Abstract

We derive the dynamics of magnetohydrodynamic waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Two-fluid magnetohydrodynamic equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear magnetohydrodynamic waves are derived for simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. We found that two- and single-fluid descriptions give similar results for low frequency waves. However, the dynamics of MHD waves in two-fluid approach is significantly changed when the wave frequency becomes comparable or higher than ion-neutral collision frequency. Alfven and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 ion-neutral collision frequency for 50 % of neutral Hydrogen) in wave spectrum. The damping rates are reduced for higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales smaller than ion-neutral collision time. Therefore, two-fluid approximation should be used for the description of relatively fast processes. Some results of single-fluid description, for example the damping of high-frequency Alfven waves in the solar chromosphere due to ion-neutral collisions, should be revised in future.