Propagating torsional Alfv\'en waves in thermally active solar plasma

Abstract

The aim of the present study is to shed light on the effects connected with thermal misbalance due to non-equal cooling and heating rates induced by density and temperature perturbations in solar active regions hosting either propagating torsional or shear Alfv\'en waves. A description for the nonlinear forces connected with Alfv\'en waves in non-ideal conditions is provided based on the second order thin flux tube approximation. This provides insight on the effects of Alfv\'en-induced motions on the boundary of thin magnetic structures in thermally active plasmas. The equations describing the process of generating longitudinal velocity perturbations together with density perturbations by nonlinear torsional Alfv\'en waves are obtained and solved analytically. It is shown that the phase shift (compared to the ideal case) and the amplitude of the induced longitudinal plasma motions against the period of mother Alfv\'en wave are greater for shear Alfv\'en waves compared to torsional Alfv\'en waves although following the same pattern. The difference in the influence of thermal misbalance on the induced velocity perturbations is governed by the plasma-beta although its effect is stronger for shear waves. It is deduced that for a harmonic Alfv\'en driver the induced density perturbations are left uninfluenced by the thermal misbalance.