Structure of plage flux tubes from the inversion of Stokes spectra. I. Spatially averaged Stokes I and V profiles

We present results of the inversion of spatially averaged Stokes I and V pr ofiles emerging from plage regions near disk center. The observations analy zed in this work were recorded with the Advanced Stokes Polarimeter. All at mospheric quantities determining the radiative transfer in the thin flux-tu be approximation are inferred self-consistently with the help of the invers ion code described by Bellot Rubio et al. With regard to thermodynamics, th e retrieved model atmospheres are found to behave as expected on theoretica l grounds. For the first time, velocities inside and outside the tubes have been derived empirically. The magnetic atmospheres resulting from the inve rsion are characterized by the absence of significant motions in high layer s but show strong velocity gradients in deeper layers. These gradients turn out to be essential for reproducing the whole shape of the observed profil es and, in particular, the asymmetries and the extended red tail of Stokes V. Our scenario predicts that the Stokes V zero-crossing wavelengths of Fe I and Fe II lines are redshifted by small but nonnegligible amounts, which is indeed confirmed by observations made with the Fourier Transform Spectro meter. According to recent numerical simulations, the internal downflows de rived from the inversion could be produced by the strong shear that takes p lace in the intermediate layer between the magnetized interior and the ambi ent medium. Another possible origin is magnetic flux undergoing convective collapse within the resolution element.