Buoyancy-driven Magnetohydrodynamic Waves in a Partially Ionized Plasma

A. Hague,R. Erdélyi

doi:10.3847/1538-4357/aae25f

Abstract

Abstract A magnetohydrodynamic (MHD) fluid description is typically employed to study the magnetized plasma comprising the solar atmosphere. This approach has had many successes in modeling and explaining solar phenomena. Most often, the plasma is assumed to be fully ionized. While this approach is justified in the higher atmosphere, i.e., the solar corona; the temperature in the lower solar atmosphere is such that a large proportion of the fluid may be electrically neutral. This begs the question: to what degree are the results derived from a fully ionized MHD description valid? In this article, we investigate the effect of partial ionization on buoyancy-driven MHD waves (the MHD analog of internal gravity waves) by applying a simplified two-fluid description. We show that previously derived results may be applied, when the fluid is weakly ionized, if the ion–neutral collision frequency is high. We derive dispersion relations for buoyancy-driven MHD waves, which include correction factors and damping rates due to ion–neutral collisions.

Highlights

Magnetohydrodynamic effects are observed in the lower solar atmosphere
The solar plasma is not composed of fully ionized hydrogen particles, rather the fluid consists of neutral hydrogen particles
The plasma acts as a single fluid, a higher collision frequency does not change this fact so the real part of the propagating wave is unaffected by the collision frequency

Summary

Introduction

Magnetohydrodynamic effects are observed in the lower solar atmosphere. Sunspots, pores, bright points, etc., show that magnetic fields do influence the dynamics of the solar plasma in the photosphere. The damping of MHD waves in partially ionized plasmas was studied by, e.g., Khodachenko et al (2004), Forteza et al (2007, 2008), Soler et al (2009a, 2009b, 2010), and Carbonell et al (2010) These works did not consider a two-fluid model; rather, they used a single-fluid description where the fluid consists of ions and neutrals. IGWs modified by magnetic fields in the lower solar atmosphere (that is, slow magnetoacoustic gravity (MAG) waves in the high-β plasma) are likely excited by turbulent motion close to the solar surface This mechanism was previously suggested by Komm et al (1991).

Derivation of the Dispersion Relation

Limiting Cases of the Dispersion Relation

Discussion and Conclusions