Damping of Alfvén waves in solar partially ionized plasmas: Effect of neutral helium in multi-fluid approach
Context. Chromospheric and prominence plasmas contain neutral atoms, which may change the plasma dynamics through collision with ions. Most of the atoms are neutral hydrogen, but a significant amount of neutral helium may also be present in the plasma with a particular temperature. Damping of MHD waves due to ion collision with neutral hydrogen has been studied well, but the effects of neutral helium are largely unknown. Aims. We aim to study the effect of neutral helium in the damping of Alfvén waves in solar, partially ionized plasmas. Methods. We consider a three-fluid magnetohydrodynamic (MHD) approximation, where one component is electron-proton-singly ionized helium and the other two components are the neutral hydrogen and neutral helium atoms. We derive the dispersion relation of linear Alfvén waves in isothermal and homogeneous plasma. Then we solve the dispersion relation and derive the damping rates of Alfvén waves for different plasma parameters. Results. The presence of neutral helium significantly enhances the damping of Alfvén waves compared to the damping due to neutral hydrogen at certain values of plasma temperature (10 000-40 000 K) and ionization. Damping rates have a peak near the ion-neutral collision frequency, but decrease for the higher part of the wave spectrum. Conclusions. The collision of ions with neutral helium atoms can be important for the damping of Alfvén waves in chromospheric spicules and in prominence-corona transition regions. © 2011 ESO.