Influence of torsional waves in solar magnetic flux tubes on spectral lines

The influence of torsional waves propagating along a thin, vertical, photospheric flux tube on Zeeman-split polarized line profiles (Stokes profiles) is investigated using a simple MHD model. In the presence of such a wave spatially resolved Stokes profiles are found to oscillate strongly in wavelength, amplitude and blue-red asymmetry. Qualitatively, torsional waves induce similar changes into the line profiles as kink waves (Ploner & Solanki 1997). The magnitude of the line parameter variation depends strongly on the observed location with respect to the flux-tube axis. The spatially averaged Stokes V and Q profiles are found to follow the torsional wave with double the wave frequency, some parameters of Stokes U fluctuate directly at the wave frequency, however. The other main feature of the spatially averaged profiles is their comparatively small reaction to the wave. The reason for the latter is that most polarized light is produced near the centre of the flux tube where, however, the torsional wave produces only weak perturbations. Temporally and spatially averaged Stokes profiles are found to be only negligibly shifted, but strongly broadened. The sign of the small remaining asymmetry is opposite in Stokes Q to that in V and U . The amplitude of the wave and the location of the flux tube on the solar disk have a strong influence on the magnitude of the perturbation of the Stokes profiles.