Torsional Alfven waves in partially ionized solar plasma: effects of neutral helium and stratification

Context. Ion-neutral collisions may lead to the damping of Alfven waves in chromospheric and prominence plasmas. Neutral helium atoms enhance the damping in certain temperature intervals, where the ratio of neutral helium and neutral hydrogen atoms is increased. Therefore, the height dependence of the ionization degrees of hydrogen and helium may influence the damping rate of Alfven waves. Aims. We aim to study the effect of neutral helium on the damping of Alfven waves in stratified, partially ionized plasma of the solar chromosphere. Methods. We consider a magnetic flux tube, which is expanded up to 1000 km height and then becomes vertical owing to merging with neighboring tubes, and study the dynamics of linear torsional Alfven waves in the presence of neutral hydrogen and neutral helium atoms. We start with a three-fluid description of plasma and subsequently derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven waves. Thin flux tube approximation allows us to obtain the dispersion relation of the waves in the lower part of tubes, while the spatial dependence of steady-state Alfven waves is governed by a Bessel-type equation in the upper parts of the tubes. Results. Consecutive derivation of single-fluid MHD equations results in a new Cowling diffusion coefficient in the presence of neutral helium, which is different from the previously used one. We find that shorter period (<5 s) torsional Alfven waves damp quickly in the chromospheric network owing to ion-neutral collision. On the other hand, longer period (>5 s) waves do not reach the transition region because they become evanescent at lower heights in the network cores. Conclusions. Propagation of torsional Alfven waves through the chromosphere into the solar corona should be considered with caution: low-frequency waves are evanescent owing to the stratification, while high-frequency waves are damped by ion-neutral collisions.