Coupling of the magnetic field and gas flows inferred from the net circular polarization in a sunspot penumbra

Abstract

We analyze penumbral fine structure using high-resolution spectropolarimetric data obtained by the Solar Optical Telescope on board the Hinode satellite. The spatial correlation between the net circular polarization (NCP) and Evershed flow is investigated in detail. Here we obtain that negative NCP structures are correlated with the Evershed flow channels in the limb-side penumbra, and that negative NCP or depressions of positive NCP are associated with the Evershed flow channels in the disk center-side of the penumbra for a negative-polarity sunspot in NOAA 10923. The positive NCP dominant in the disk center-side penumbra is essentially attributed to interflow channels instead of Evershed flow channels. The stratification of magnetic field and velocity are investigated by using SIR-JUMP inversion with a one-component atmosphere, and the NCP of spectral lines in the limb-side and disk center-side of the penumbra is successfully reproduced. The inversion results show that an increased Evershed flow is associated with a strong magnetic field located in the deep photosphere. Our result does not match with the simple two-component penumbral models in which the penumbra consists of Evershed flow and interflow channels and the global NCP is attributed only to the Evershed flow channels.