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

Abstract

Context. Partially ionized plasma is usually described by a single-fluid approach, where the ion-neutral collision effects are expressed by Cowling conductivity in the induction equation. However, the single-fluid approach is not valid for time-scales less than ion-neutral collision time. For these time-scales the two-fluid description is the better approximation. Aims. We aim to derive the dynamics of magnetohydrodynamic (MHD) waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Methods. Two-fluid equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear waves are derived for the simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. Results. We found that two-and single-fluid descriptions give similar results for low-frequency waves. However, the dynamics of MHD waves in the two-fluid approach is significantly changed when the wave frequency becomes comparable with or higher than the ion-neutral collision frequency. Alfvén and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 times the ion-neutral collision frequency for 50% of neutral hydrogen) in the wave spectrum. The damping rates are reduced for the higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. Conclusions. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales shorter than ion-neutral collision time. Therefore, the two-fluid approximation should be used for the description of relatively fast processes. Some results of the single-fluid description should be revised in future such as the damping of high-frequency Alfvén waves in the solar chromosphere due to ion-neutral collisions.

Rayleigh-Taylor instability in partially ionized compressible plasmas: One fluid approach

“…Since prominences are relatively cool and dense objects, their plasma is expected to be partially ionized (Patsourakos & Vial 2002;Gilbert et al 2007;Labrosse et al 2010;Zaqarashvili et al 2011b). The ionization degree of the prominence plasma has not been directly measured with accuracy but the plasma is neither in a completely ionized state or is a neutral gas with the typical physical parameters of this objects.…”

Modeling of inelastic collisions in a multifluid plasma: Excitation and deexcitation

“…Multifluid models are commonly used in simulations of astrophysical plasmas (see for example 21,22 ).…”

Alfvén ionization in an MHD-gas interactions code

“…[8][9][10] Single fluid models of partially ionized plasmas have been shown to be insufficient in certain regimes. Zaqarashvili et al 11 show that a two-fluid model is necessary when studying time-scales shorter than the ion-neutral collision time.…”