Physical properties of the solar magnetic photosphere under the MISMA hypothesis III: sunspot at disk center

Abstract

Small-scale fluctuations of magnetic field and velocity may be responsible for the Stokes asymmetries observed in all photospheric magnetic structures (the MISMA hypothesis). We support the hypothesis showing that atmospheres with optically-thin micro-structure reproduce the polarization of FeI 6301 A and FeI 6302 A observed in a sunspot. Ten thousand spectra were fitted by model MISMAs with two magnetic components interleaved along the line-of-sight. Combining all the fits, we set up a semi-empirical model sunspot characterized by two components with very different magnetic field inclination. The major component, which contains most of the mass, is more vertical than the minor component. The field lines of the minor component are inclined below the horizontal plane throughout the penumbra. Magnetic field lines and mass flows are parallel, consequently, both upflows and downflows are present everywhere on the penumbra. Major and minor components have very different velocities (several hundred m/s for the major component versus 10 km/s for the minor component), but the mass transported per unit time is similar. The similarity between the vertical mass flow and the magnetic flux of the two components suggests that field lines emerging as major component may return to the photosphere as minor component. If so, the observed magnetic field strength difference between components leads to a siphon flow whose magnitude and direction agree with the inferred Evershed flow. Several tests support the internal consistency of the retrieved model sunspot. The magnetic field vector {B} does not violate the {nabla {B}=0} condition. The model sunspot reproduces the net circular polarization of the observed lines, plus the abnormal behavior of FeI 15648 A.