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 λ 6173 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.
Highlights
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
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
We could clearly see that the light bridges (LBs) with their dividing dark lanes are remarkable and that the penumbra had developed only rudimentarily (Schlichenmaier et al 2010a,b)
Summary
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. 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. Numerical simulations of the formation of an active region were performed by, e.g., Cheung et al (2010, see 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. 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. 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
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
