Alfven Waves in Partially Ionised Solar Steady-State Plasmas

TL;DR

This study analyzes Alfvén waves in partially ionised solar plasmas with field-aligned steady flows for ions and neutrals using a two-fluid MHD framework. It derives a dispersion relation expressed in dimensionless form with $X=\nu_{in}/(k c_A)$, $Y=\omega/(k c_A)$, and Mach numbers $M_i$, $M_n$, along with the ionisation fraction $\chi$, yielding a cubic equation in $Y$ that generalises previous static results. Numerical solutions reveal three modes (forward and backward Alfvén waves and a flow mode driven by neutral advection) whose frequencies and damping depend on collisional coupling and ionisation level, with flow-induced asymmetries and possible mode conversion. The results have implications for diagnosing neutral flows and energy transfer in the solar chromosphere, while highlighting limitations such as the assumption of homogeneous flows and the neglect of Cowling resistivity, pointing to future work on shear flows and additional wave modes.

Abstract

Our study investigates the properties of Alfvén waves in partially ionised solar plasmas in the presence of steady, field-aligned, flows of charged and neutral particles. Our work aims to understand how such flows modify wave propagation and damping in environments where ion-neutral collisions are significant. We employ a two-fluid model that treats ions and neutrals as separate, colliding fluids and incorporates background steady flows for both species. Using a combination of analytical dispersion analysis and numerical solutions, we examine the impact of these flows on the behaviour of Alfvén waves. Our results show that steady flows lead to substantial modifications of wave properties, including Doppler shifts, propagation direction reversal, flow-dependent changes in damping rates, and the appearance of a new mode associated with neutral flow and collisional coupling. We also identify conditions under which flow-driven mode conversion can arise. Our results offer new insights into the interplay between plasma flows and particle collisions in the regions of the solar atmosphere where partial ionisation is relevant.

Figures (6)

• Figure 1: The real (left column) and imaginary (right column) parts of $Y$ for weakly ionised ($\chi =2$, top row) and strongly ionised ($\chi =0.2$, bottom row) plasmas in a static equilibrium $( M_{i}=M_{n}=0)$. The variation of $Y$ is represented as a function of the dimensionless variable, $X$. The forward (F) and backwards (B) propagating Alfvén waves are shown by green and blue lines, respectively. The mode shown in red is the non-propagating (evanescent) entropy mode.
• Figure 2: Special case of mode conversion for $\chi =0.2$, and $M_n=-1.1$. The two panels show the real (left-hand) and imaginary (right-hand) parts of $Y$, highlighting mode conversion and damping.
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