  We investigate the spatial distribution of magnetic polarities in the
penumbra of a spot observed very close to disk center. High-spatial resolution,
high-cadence magnetograms taken with the Narrowband Filter Imager aboard Hinode
are used in this study. They provide continuous and stable measurements in the
photospheric Fe I 630.25 line for long periods of time. We discover
small-scale, elongated, bipolar magnetic structures that appear in the mid
penumbra and move radially outward across the penumbra. They occur in between
the more vertical fields of the penumbra, and can be associated with the
horizontal fields that harbor the Evershed flow. Many of them cross the outer
penumbral boundary, becoming moving magnetic features in the sunspot moat. We
determine the properties of these structures, including their sizes, proper
motions, footpoint separation, and lifetimes. The bipolar patches can be
interpreted as being produced by sea-serpent field lines that originate in the
mid-penumbra and eventually leave the spot in the form moving magnetic
features. The existence of such field lines has been inferred from Stokes
inversions of spectropolarimetric measurements at lower angular resolution, but
this is the first time they are imaged directly. Our observations add another
piece of evidence in favor of the uncombed structure of penumbral magnetic
fields.
