  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.
