Damping of oscillations by ion-neutral collisions in a prominence plasma

¹ Departament de Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain e-mail: [pep.forteza;ramon.oliver;dfsjlb0]@uib.es
² Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria e-mail: maxim.khodachenko@oeaw.ac.at

Received: 26 June 2006
Accepted: 29 September 2006

Abstract

Aims.The role of collisions between ions, electrons and neutrals in a partially ionised plasma is assessed as a possible wave damping mechanism. The relevance of this mechanism in the damping of small amplitude prominence oscillations is evaluated.

Methods.A one-fluid MHD set of equations taking into account various effects in a partially ionised solar plasma (collisions between different species and Joule dissipation) is derived. Assuming small perturbations, these equations are next linearised about a uniform equilibrium configuration and the dispersion relation of magnetoacoustic waves in an unbounded medium is obtained.

Results.The presence of neutrals in the plasma only affects the fast wave in a relevant way. An approximate expression for the damping rate is obtained which shows that the strongest damping takes place in a medium with strong magnetic field, low density and low ionisation fraction. Wave attenuation arises mostly from collisions between ions and neutrals.

Conclusions.Given the poor knowledge about the values of the density and ionisation fraction in prominences, it is hard to judge the importance of the physics of partial ionisation in the damping of fast waves in solar prominences. Nevertheless, note that a very idealised case, with no stratification and no equilibrium currents, is considered here, so the addition of these features to the model may change the results of this work.