Anomalous flows in a sunspot penumbra

Context. The photospheric Evershed flow is a distinct characteristic of penumbrae that is closely associated with the photometric and magnetic structure of sunspots. Aims. We analyze the properties of an anomalous flow in the photosph ere in a sunspot penumbra and compare it with those of the regular Evershed flow. Methods. High-resolution spectropolarimetric observations of act ive region NOAA 11271 were obtained with the spectro-polarimeter (SP) on board Hinode. We use the magnetic field properties derived by a Milne-Eddi ngton inversion. In addition, we use Ca II H images obtained by the broad-band filter instrument to study the low chromospheric response to the photospheric structure and Dopplergrams from the Helioseismic and Magnetic Imager to follow the evolution of the photospheric flows. Results. We detect a blue-shifted feature that appeared on the limb-side penumbra of a sunspot and that was present intermittently during the next two hours. It exhibited a maximum blue-shift of 1.6 km s −1 , an area of 5.2 arcsec 2 , and a maximum uninterrupted lifetime of 1 hr. The blue-shifted feature, when present, lies parall el to red-shifts. Both blue and red shifts flank a highly incli ned/horizontal magnetic structure that is radially oriented in the penumbr a. The low-cadence SP maps reveal changes in size, radial position in the penumbra and line-of-sight (LOS) velocity of the blue-shifted feature, from one scan to the other. There was an increase of nearly 500 G in the field strength with the onset of the blue-shifts, p articularly when the LOS velocity in the feature exceeded 1.5 km s −1 . There was only a marginal reduction in the field inclination o f about 10 ◦ with the increase in blue-shifts. In the chromosphere, inte nse, arc-shaped brightenings were observed close to the location of the photospheric blue-shifts, which extend from the edge of the umbral core to the penumbra-quiet Sun boundary. The intensity of these brightenings exceeds the background intensity by 30 to 65% with the strongest and largest brightenings observed about 30 min after the strongest blue-shifts were detected at the photosph ere. The close spatial proximity of the two phenomenon strongly suggests a causal relationship. Conclusions. The blue-shifted feature represents plasma motion that could be related to a magnetic structure that rises in the solar atmosphere and subsequently reconnects with the ambient chromospheric magnetic field of the sunspot or an inverse Evers hed flow, which would be unique in the photosphere. This transient phenomena is presumably related to the dynamic stability of the sunspot because the corresponding umbral core separated two days later at the location of the blue-shifts and fragmented subsequently.