Brightness, distribution, and evolution of sunspot umbral dots

Riethmüller, T. L.; Solanki, S. K.; Zakharov, V.; Gandorfer, A.

doi:10.1051/0004-6361:200810701

Cited by 46 publications

(72 citation statements)

References 35 publications

(68 reference statements)

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“…5). We note that UD proper motions from 0.4 km s −1 to 1.3 km s −1 have been reported by several authors (e.g., Riethmüller et al 2008;Watanabe et al 2010;Sobotka & Puschmann 2009). They have been found to be aligned mainly towards the umbral centre or along light bridges (LBs).…”

Section: Discussionsupporting

confidence: 83%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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Shear and vortex motions in a forming sunspot

González

Kneer²,

Schlichenmaier

2012

A&A

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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.

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Section: Discussionsupporting

confidence: 83%

Section: Introductionmentioning

confidence: 99%

Shear and vortex motions in a forming sunspot

González

Kneer²,

Schlichenmaier

2012

A&A

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“…These values are higher than those reported in recent observational studies of UDs with the 1-m SST on La Palma: Riethmüller et al (2008b) give an average area of 0.04 Mm 2 , Sobotka & Hanslmeier (2005) estimate 0.025 Mm 2 while Sobotka & Puschmann (2009) even find a value near 0.01 Mm 2 . On the other hand, the UDs with dark lanes studied by the latter authors have an average area around 0.08 Mm 2 and even larger UDs have been studied by Bharti et al (2007b).…”

Section: Statistical Properties Of Umbral Dotscontrasting

confidence: 70%

“…Red and green dots refers to datasets P1 and P2, respectively; solid black (P1) and blue (P2) lines connect binned values for 50 points each. Kitai et al 2007;Riethmüller et al 2008b;Sobotka & Puschmann 2009;Watanabe et al 2009a). However, similar to our simulation results, the distribution of lifetimes from any observation is rather broad and "typical" lifetimes cannot be defined easily (Socas-Navarro et al 2004).…”

Section: Statistical Properties Of Umbral Dotsmentioning

confidence: 99%

Properties of simulated sunspot umbral dots

Bharti¹,

Beeck²,

Schüßler³

2010

A&A

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Realistic 3D radiative MHD simulations reveal the magneto-convective processes underlying the formation of the photospheric fine structure of sunspots, including penumbral filaments and umbral dots. Here we provide results from a statistical analysis of simulated umbral dots and compare them with reports from high-resolution observations. A multi-level segmentation and tracking algorithm has been used to isolate the bright structures in synthetic bolometric and continuum brightness images. Areas, brightness, and lifetimes of the resulting set of umbral dots are found to be correlated: larger umbral dots tend to be brighter and live longer. The magnetic field strength and velocity structure of umbral dots on surfaces of constant optical depth in the continuum at 630 nm indicate that the strong field reduction and high velocities in the upper parts of the upflow plumes underlying umbral dots are largely hidden from spectro-polarimetric observations. The properties of the simulated umbral dots are generally consistent with the results of recent highresolution observations. However, the observed population of small, short-lived umbral dots is not reproduced by the simulations, possibly owing to insufficient spatial resolution.

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“…A number of studies modeling the rise of thin flux tubes through the convection zone have also shown that the tubes must have a significant amount of twist in order to maintain their integrity and not fragment in the face of hydrodynamic forces, and indeed observations show that magnetic flux usually emerges at the surface already in a significantly twisted state (e.g. Riethmüller et al, 2008).…”

Section: Flux Emergencementioning

confidence: 99%

Modeling the Subsurface Structure of Sunspots

et al. 2010

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While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009aGizon et al. ( , 2009b. We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.

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Brightness, distribution, and evolution of sunspot umbral dots

Cited by 46 publications

References 35 publications

Shear and vortex motions in a forming sunspot

Shear and vortex motions in a forming sunspot

Properties of simulated sunspot umbral dots

Modeling the Subsurface Structure of Sunspots

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