Radiation transfer through a model sunspot

A cylindrically symmetric model for a sunspot atmosphere using the similarity principle of Schlueter and Temesvary for the magnetic field configuration is presented. The equations of magnetostatic equilibrium are used, augmented by a radial Evershed flow. The LTE radiative transfer equations for the Stokes vector were solved under a variety of conditions for a ray emerging from a typical penumbral point. The contribution from isolated lines to the broadband circular polarization in sunspot penumbrae is evaluated using a more realistic model sunspot atmosphere than has hitherto been considered. Results indicate that the inclusion of a velocity field along the magnetic field vector is unable to give a net circular polarization of sufficient magnitude, although the variation with the angle between the line-of-sight and the magnetic field vector is in qualitative agreement with observations. The corresponding results for the net linear polarization are satisfactory.