Alfven waves in a partially ionized two-fluid plasma

TL;DR

This paper investigates how Alfvén waves behave in partially ionized plasmas using a two-fluid model, analyzing wave modifications, attenuation, and implications for solar atmospheric energy transfer.

Contribution

It provides a comprehensive theoretical analysis of Alfvén waves in partially ionized plasmas, including wave frequency modifications, attenuation effects, and the impact of ionization degree and collisions.

Findings

Neutral-ion collisions modify wave frequencies.

Existence of wavelength cut-offs for oscillatory waves.

Partial ionization affects energy flux in solar plasma.

Abstract

Alfv\'en waves are a particular class of magnetohydrodynamic waves relevant in many astrophysical and laboratory plasmas. In partially ionized plasmas the dynamics of Alfv\'en waves is affected by the interaction between ionized and neutral species. Here we study Alfv\'en waves in a partially ionized plasma from the theoretical point of view using the two-fluid description. We consider that the plasma is composed of an ion-electron fluid and a neutral fluid, which interact by means of particle collisions. To keep our investigation as general as possible we take the neutral-ion collision frequency and the ionization degree as free parameters. First, we perform a normal mode analysis. We find the modification due to neutral-ion collisions of the wave frequencies and study the temporal and spatial attenuation of the waves. In addition, we discuss the presence of cut-off values of the wavelength that constrain the existence of oscillatory standing waves in weakly ionized plasmas. Later, we go beyond the normal mode approach and solve the initial-value problem in order to study the time-dependent evolution of the wave perturbations in the two fluids. An application to Alfv\'en waves in the low solar atmospheric plasma is performed and the implication of partial ionization for the energy flux is discussed.