Volume 549, January 2013
|Number of page(s)||9|
|Published online||08 January 2013|
Torsional Alfvén waves in partially ionized solar plasma: effects of neutral helium and stratification
Space Research Institute, Austrian Academy of Sciences,
e-mail: email@example.com; firstname.lastname@example.org
2 Departament de Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
3 Abastumani Astrophysical Observatory at Ilia State University, University St. 2, 1060 Tbilisi, Georgia
Accepted: 14 November 2012
Context. Ion-neutral collisions may lead to the damping of Alfvén 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 Alfvén waves.
Aims. We aim to study the effect of neutral helium on the damping of Alfvén 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 Alfvén 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 Alfvén 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 Alfvén 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 Alfvén 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 Alfvén 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.
Key words: Sun: atmosphere / Sun: oscillations
© ESO, 2013
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.