NET CIRCULAR POLARIZATION OF SUNSPOT PENUMBRAE - SYMMETRY BREAKING BY ANOMALOUS DISPERSION

摘要

We examine the polarization of spectral lines in the penumbra of sunspots by solving the radiative transfer equation of polarized light for a three-dimensional axially symmetric model atmosphere of a sunspot. The Evershed flow is confined to horizontal magnetic flux tubes obtained from MHD calculations. These are embedded in an inclined background magnetic field. In this work, we concentrate on the Stokes-V profiles and examine the net circular polarization (NCP), N = ∫V(λ)dλ, of two photospheric spectral lines of neutral iron, Fe I 630.25nm and Fe I 1564.8 nm. Analyzing spectra at a fixed distance from the spot center, we find that the azimuthal variation of N, N(ψ), is an antisymmetric function of ψ w.r.t. to the line connecting disk center and spot center for Fe I 1564.8nm, while the variation is predominantly symmetric for Fe I 630.25 nm. We show that the antisymmetric variation is caused by anomalous dispersion (rotation of the polarization vector in a magnetized plasma). The different inclination angles lead to a discontinuity in the azimuth of the magnetic field along the line-of-sight. We show that this discontinuity together with the effect of anomalous dispersion produces an antisymmetric component in N(ψ) which outweighs the symmetric component from the discontinuity for Fe I 1564.8 nm, while it is negligible for Fe I 630.25 nm. We finally compute synthetic NCP maps of a sunspot which offer an explanation for recent observational results.