Anomalous flows in a sunspot penumbra

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: rlouis@aip.de
² National Solar Observatory, Sacramento Peak, 3010 Coronal Loop, Sunspot, New Mexico 88349, USA
³ Udaipur Solar Observatory, Physical Research Laboratory, Dewali, Badi Road, 313004 Udaipur, Rajasthan, India

Received: 1 May 2014
Accepted: 27 August 2014

Abstract

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 analyse the properties of an anomalous flow in the photosphere in a sunspot penumbra and compare it with those of the regular Evershed flow.

Methods. High-resolution spectropolarimetric observations of active region NOAA 11271 were obtained with the spectro-polarimeter (SP) on board Hinode. We used the magnetic field properties derived by a Milne-Eddington inversion. In addition, we used 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 blueshifted 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 blueshift of 1.6 km s^-1, an area of 5.2 arcsec², and a maximum uninterrupted lifetime of 1 h. The blueshifted feature, when present, lies parallel to redshifts. Both blue- and redshifts flank a highly inclined or horizontal magnetic structure that is radially oriented in the penumbra. The low-cadence SP maps reveal changes in size, radial position in the penumbra, and line-of-sight (LOS) velocity of the blueshifted feature, from one scan to the next. There was an increase of nearly 500 G in the field strength with the onset of the blueshifts, particularly when the LOS velocity in the feature exceeded 1.5 km s^-1. There was only a marginal reduction in the field inclination of about 10° with the increase in blueshifts. In the chromosphere, intense, arc-shaped brightenings were observed close to the location of the photospheric blueshifts, 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 blueshifts were detected at the photosphere. The close spatial proximity of the two phenomenon strongly suggests a causal relationship.

Conclusions. The blueshifted 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 Evershed 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 blueshifts and fragmented subsequently.