5 Conclusion

We demonstrate that a discontinuity in the azimuth, $\triangle \phi$, of the magnetic field vector along the line-of-sight together with the effects of anomalous dispersion plays a crucial role for the interpretation of spectropolarimetric measurements in sunspot penumbrae.

In an axially symmetric sunspot in which the magnetic and velocity field vectors have no azimuthal components, a nearly horizontal flow channel embedded in an inclined magnetic background field introduces a discontinuity, $\triangle \phi$, in the azimuth relative to the line-of-sight. Along an azimuthal section within the penumbra, $\triangle \phi (\psi )$ is antisymmetric w.r.t. the line-of-symmetry, giving rise to an antisymmetric contribution to the net circular polarization, $N(\psi )$. $N(\psi )$ consists of a symmetric contribution from $\triangle \gamma$ (and $\triangle B$ which, however, is negligible in our model configuration) and an antisymmetric contribution from $\triangle \phi$. The wavelength shift ratio between the Doppler and the Zeeman effect determines the relative weights of these two contributions to N. The difference between the symmetry properties of N-maps in Fe I 1564.8 nm and Fe I 630.2 nm can therefore be attributed to the large wavelength difference between the two lines.

The striking difference between observed N-maps for the Fe I 1564.8 and the Fe I 630.2 line can be reproduced by synthetic lines that emanate from a model atmosphere which is based on the moving tube model of SJS98. In this respect, the present work provides strong evidence that magnetic fields with (at least) two different inclinations with different flow velocities are present in the penumbra. It also demonstrates that the spatial distribution of $N(r,\psi)$ within the penumbra is a valuable diagnostic tool in order to test penumbral models.