Structure of proper motions in a sunspot penumbra

We study the structure and evolution of the horizontal proper motions in a regular sunspot penumbra, very close to the solar disc center, in active region NOAA 11092 using a 48 min time sequences of blue continuum images recorded by Hinode/SOT in 2010 August 3. We apply local correlation tracking (LCT). The penumbra shows a slow (fast) flow field with an average speed of 0.2 (0.4) km/s starting at its middle towards the umbra (outer penumbral boundary) as an inward (outward) motion in accordance with previous findings. This behavior defines a continuous divergence line at the middle of the penumbra (r~2R_spot/3). A distorted ringlike feature with very slow flows (~50 m/s; zero-flow ring: ZFR) co-spatial with the divergence line is clearly seen. Deep intrusion of coordinated penumbral filaments into the umbra can cause the ZFR to be a) significantly displaced towards the umbra or b) discontinuous, showing considerable speeds there (~150 m/s). Where the ZFR shows discontinuity, the divergence line does not move toward the umbra. Also, because of the different evolutionary flows of adjacent penumbral filaments, the ZFR and the divergence line show a stable backward/forward displacement along itself during the 48 min observation. The radial variations of the azimuthally averaged brightness show a local bright ring with a weak contrast of 1% close to the ZFR. At the outer penumbra, we find that the converging filamentary flow occurs in a dark radial channel and the filamentary diverging flows are formed by the evolution of thin bright fibrils. Also, the large speeds at the penumbra boundary are produced by the displacement and/or the fragmentation of the bright fibrils in developing filamentary flows. In surrounding granulation, some divergence centers are strongly pushed away as a whole with an average speed of about 0.6 km/s by these developing filamentary flows.