The Fine-Structure of the Net-Circular Polarization in a Sunspot Penumbra

TL;DR

This study reveals a filamentary fine structure in the net-circular polarization of a sunspot penumbra, providing new insights into magnetic field configurations and supporting the uncombed penumbra model.

Contribution

First detection of filamentary NCP fine structure in a sunspot penumbra and interpretation using radiative transfer modeling of embedded magnetic flux tubes.

Findings

Detection of filamentary NCP structure on penumbral scales

Observation of zero-crossing of NCP in outer penumbra

Explanation of zero-crossing via magnetic flux tube field variations

Abstract

We present novel evidence for a fine structure observed in the net-circular polarization (NCP) of a sunspot penumbra based on spectropolarimetric measurements utilizing the Zeeman sensitive FeI 630.2 nm line. For the first time we detect a filamentary organized fine structure of the NCP on spatial scales that are similar to the inhomogeneities found in the penumbral flow field. We also observe an additional property of the visible NCP, a zero-crossing of the NCP in the outer parts of the center-side penumbra, which has not been recognized before. In order to interprete the observations we solve the radiative transfer equations for polarized light in a model penumbra with embedded magnetic flux tubes. We demonstrate that the observed zero-crossing of the NCP can be explained by an increased magnetic field strength inside magnetic flux tubes in the outer penumbra combined with a decreased magnetic field strength in the background field. Our results strongly support the concept of the uncombed penumbra.