Nonlinear coupling of Alfén and magnetoacoustic waves in partially ionized solar plasmas

Abstract

[eng] Context. Partially ionized plasmas constitute an essential ingredient of the solar atmosphere since layers such as the chromosphere and the photosphere and structures such as prominences and spicules are made of this plasma. On the other hand, ground- and space-based observations have indicated the presence of oscillations in partially ionized layers and structures of the solar atmosphere, which have been interpreted in terms of magnetohydrodynamic (MHD) waves. Aims. Our aim is to study the temporal behavior of nonlinear Alfvén waves, and the subsequent excitation of field-aligned motions and perturbations, in a partially ionized plasma when dissipative mechanisms such as ambipolar diffusion, radiative losses, and thermal conduction are taken into account. Methods. First, we applied the regular perturbations method for small-amplitude initial perturbations to obtain the temporal behavior of perturbations. Then we solved the full set of nonlinear MHD equations for larger values of the initial amplitude. Results. We obtain analytical and numerical solutions to first-, second-, and third-order systems of equations and study the effects produced by ambipolar diffusion and thermal mechanisms on the temporal behavior of Alfvén and slow waves. We also study how the majority of the energy is transferred from the Alfvén waves to plasma internal energy. After numerically solving the full nonlinear equations when a large amplitude is assumed, the profile of the perturbations displays the typical sawtooth profile characteristic of associated shocks. Conclusions. When ambipolar diffusion is taken into account, first-order Alfvén waves are damped in time, while second-order perturbations are undamped. However, due to the release of heat produced by ambipolar diffusion, other physical effects that modify the physical conditions in the spatial domain under consideration appear. On the other hand, the second-order perturbations are damped by thermal effects with a damping time that can be longer or shorter than that of Afvén waves. Therefore, after the initial excitation, Alfvén waves can be quickly damped, while slow waves remain in the plasma for a longer time, and vice versa.