Effect of partial ionization on wave propagation in solar magnetic flux tubes

TL;DR

This paper investigates how partial ionization affects MHD wave propagation in solar flux tubes, revealing modifications in wave frequencies, damping effects, and conditions for wave propagation or cutoff in partially ionized plasma.

Contribution

It provides a theoretical analysis of wave behavior in partially ionized solar plasma using a two-fluid model, including derivation of dispersion relations and effects of neutral-ion collisions.

Findings

Wave frequencies depend on ionized fluid properties at low collision frequencies.

High collision frequencies cause ions and neutrals to behave as a single fluid with modified wave properties.

Neutral gas pressure influences slow magnetoacoustic wave behavior and damping.

Abstract

Observations show that waves are ubiquitous in the solar atmosphere and may play an important role for plasma heating. The study of waves in the solar corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully ionized plasma. However, the plasma in the photosphere and the chromosphere is only partially ionized. Here we investigate theoretically the impact of partial ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid model. We derive the general dispersion relation that takes into account the effects of neutral-ion collisions and the neutral gas pressure. We take the neutral-ion collision frequency as an arbitrary parameter. Particular results for transverse kink modes and slow magnetoacoustic modes are shown. We find that the wave frequencies only depend on the properties of the ionized fluid when the neutral-ion collision frequency is much lower that the wave frequency. For high collision frequencies realistic of the solar atmosphere ions and neutrals behave as a single fluid with an effective density corresponding to the sum of densities of both fluids and an effective sound velocity computed as the average of the sound velocities of ions and neutrals. The MHD wave frequencies are modified accordingly. The neutral gas pressure can be neglected when studying transverse kink waves but it has to be taken into account for a consistent description of slow magnetoacoustic waves. The MHD waves are damped due to neutral-ion collisions. The damping is most efficient when the wave frequency and the collision frequency are of the same order of magnitude. For high collision frequencies slow magnetoacoustic waves are more efficiently damped than transverse kink waves. In addition, we find the presence of cut-offs for certain combinations of parameters that cause the waves to become non-propagating.