Shear and vortex motions in a forming sunspot - Twist relaxation in magnetic flux ropes

Astronomy & Astrophysics A&A 538, A62 (2012) DOI: 10.1051/0004-6361/201118005 c ESO 2012  Shear and vortex motions in a forming sunspot Twist relaxation in magnetic flux ropes N. Bello González1 , F. Kneer2 , and R. Schlichenmaier1 1 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: [nbello;schliche]@kis.uni-freiburg.de 2 Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: Received 2 September 2011 / Accepted 28 October 2011 ABSTRACT Aims. We measure proper motions of fine structures in a forming sunspot to infer information about the dynamics of flux emergence at the sub-photospheric level. Methods. The active region NOAA 11024 was observed with the Vacuum Tower Telescope at Observatorio del Teide/Tenerife over several days in July 2009. Here, we concentrate on a two-hour sequence taken on July 4, when the leading spot was at an early stage of its evolution. Speckle reconstructions from Ca ii K images and polarimetric data in Fe i λ6173 allow us to study proper motions of umbral fine structures. Results. We detect three prominent features: (1) A light bridge, divided by a dark lane along its axis, shows proper motions in opposing directions on its sides, with velocities of ∼100–500 m s−1 . The flows are seen in both the Ca ii K and the broadband time sequences. (2) Umbral dots in one umbral region outline a vortex with speeds of up to 550 m s−1 . The direction of the motion of the umbral dots is different from that in the light bridge. (3) At one rim of the umbra, the fine structure of the magnetic field moves horizontally with typical velocities of 250–300 m s−1 , prior to the formation of the penumbra. Conclusions. We report on shear and vortex motions in a forming sunspot and interpret them as tracers of twist relaxation in magnetic flux ropes. We suggest that the forming sunspot contains detached magnetic flux ropes that emerge at the surface with different amounts of twist. As they merge to form a sunspot, they untwist giving rise to the observed shear and vortex motions. Key words. techniques: spectroscopic – Sun: activity – sunspots – techniques: high angular resolution 1. Introduction The observational properties of fine structures in sunspots remain one of the focci of solar physics research, since they are thought to be the key to understanding the physics of sunspots. In the present study, we report on proper motions of small-scale features in the umbra and a light bridge of a sunspot at its early stage of formation, and we discuss how these motions can be related to the evolution of magnetic flux ropes and their reorganisation during their emergence from the convection zone. Parker (1979) suggested that the sunspot magnetic field divides into many separate tubes approximately 1 Mm below the photosphere. The bright umbral dots (UDs) and light bridges (LBs), might then represent the occasional upward intrusion of the field-free gas between the flux tubes to the surface. Schüssler & Vögler (2006) carried out realistic three-dimensional magnetohydrodynamic simulations of a plasma that is representative of sunspot umbrae. The simulations of magneto-convection in a monolithic magnetic field reaching 1.2 Mm below the surface produced UDs that are very similar to those observed in terms of brightness, size, and lifetime. Numerical simulations of the formation of an active region were performed by, e.g., Cheung et al. (2010, see also their references to other work). They allowed a vertical semi-torus of a twisted magnetic flux rope to advect into their simulation box reaching 7.5 Mm below the solar surface and studied the evolution during emergence. In the course of its rise, the flux rope expands and splits into many flux tubes that emerge through the surface in a large patch. Later on, the tubes combine to form two spots of opposite polarity that rotate owing to the field twist. Remnants of hot gas with strongly reduced field strength between the re-combined flux tubes are seen as LBs at the surface. Riethmüller et al. (2008) carried out a detailed observational study of the morphology, evolution, and substructures of UDs. They find proper motions of UDs mainly from the umbral border inward with typical velocities of 420 m s−1 and preferred motions of UDs away from an LB on one side and towards the LB on the other side. Watanabe et al. (2010) detected a UD that moved rapidly, with 1.3 km s−1 , towards the umbra centre. Sobotka & Puschmann (2009) also found motions of UDs, directed either into the umbra or along a faint LB. In addition, these authors, as well as Ortiz et al. (2010) from spectropolarimetric observations, confirm with high spatial resolution data, the existence of dark lanes within UDs, as predicted by Schüssler & Vögler (2006). Further measurements of proper motions of bright structures in LBs in a pore were performed by Hirzberger et al. (2002). They found irregular motions of grains with velocities of up to 1.5 km s−1 . Although they did not discuss this, persistent flows of about half-hour duration can be seen in their Fig. 8. Berger & Berdyugina (2003) presented unidirectional flows of small-scale grains in an LB with an average speed of 900 m s−1 . And Rimmele (2008), using a local correlation tracking algorithm, measured proper motions in an LB with speeds of up to Article published by EDP Sciences A62, page 1 of 5 A&A 538, A62 (2012) 580 m s−1 . This flow persists for at least 24 h. Rimmele noted that “the flow vectors indicate that the flow ends in a vortex” and pointed towards a local mechanism as the possible driver of the observed horizontal flows. 2. Observations and data analysis Our data stem from observations of the active region (AR) NOAA 11024 during July 1–10, 2009 with the Vacuum Tower Telescope at Observatorio del Teide/Tenerife. Schlichenmaier et al. (2010a,b) and Rezaei et al. (2012) described the observational details and reported on the evolution of the AR, on the formation of the spot and on its magnetic properties. We concentrated on a 1 h 53 min time series, from 08:31 UT to 10:24 UT on July 4, 2009, i.e., during the early stages of the evolution of the AR leading sunspot. Its position on the Sun was 6◦ E, 25◦ S, cos θ = 0.88, with heliocentric angle θ. The data used consisted of: (1) Ca ii K bursts taken through a filter with FWHM 1 nm; (2) simultaneous speckle observations in Fe i λ6173 (Landé factor g = 2.5) with the two-dimensional “Göttingen” Fabry-Perot spectrometer (GFPI, since fall 2009 GREGOR FPI) with its full Stokes polarimetric capability (Bendlin et al. 1992; Puschmann et al. 2006; Bello González & Kneer 2008). Together with the FPI narrow-band frames, broadband burst observations around 6300 Å (FWHM 50 Å) were taken. The Ca ii K frames were reconstructed with the speckle interferometry package KISIP (Wöger et al. 2008). The FPI observations were reconstructed with the Göttingen speckle code (de Boer 1996; Janssen 2003; Bello Gon (...truncated)