Resonant Alfven waves in partially ionized plasmas of the solar atmosphere

TL;DR

This study investigates how resonant Alfvén waves in partially ionized solar atmospheric plasmas are damped and how they transfer energy, revealing that resonant absorption dominates damping over ion-neutral collisions at observed frequencies.

Contribution

The paper provides analytical and numerical analysis of resonant Alfvén waves in partially ionized flux tubes, highlighting the damping mechanisms and energy transfer processes in the solar chromosphere.

Findings

Resonant absorption damping length is inversely proportional to frequency.

Ion-neutral collisions damp high-frequency waves more effectively.

Most wave energy converts into localized Alfvén waves depositing energy in the corona.

Abstract

Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. In magnetic waveguides resonant absorption due to plasma inhomogeneity naturally transfers wave energy from large-scale motions to small-scale motions. In the cooler parts of the solar atmosphere as, e.g., the chromosphere, effects due to partial ionization may be relevant for wave dynamics and heating. Aims. We study resonant Alfven waves in partially ionized plasmas. Methods. We use the multifluid equations in the cold plasma approximation. We investigate propagating resonant MHD waves in partially ionized flux tubes. We use approximate analytical theory based on normal modes in the thin tube and thin boundary approximations along with numerical eigenvalue computations. Results. We find that the jumps of the wave perturbations across the resonant layer are the same as in fully ionized plasmas. The damping length due to resonant absorption is inversely proportional to the frequency, while that due to ion-neutral collisions is inversely proportional to the square of the frequency. For observed frequencies in the solar atmosphere, the amplitude of MHD kink waves is more efficiently damped by resonant absorption than by ion-neutral collisions. Conclusions. Most of the energy carried by chromospheric kink waves is converted into localized azimuthal Alfven waves that can deposit energy in the coronal medium. The dissipation of wave energy in the chromosphere due to ion-neutral collisions is only effective for high-frequency waves. The chromosphere acts as a filter for kink waves with periods shorter than 10 s.