Evidence for Magnetoconvection in Sunspot Umbral Dots

Bharti, L.; Jain, Rajmal; Jaaffrey, S. N. A.

doi:10.1086/521024

Cited by 39 publications

(36 citation statements)

References 22 publications

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“…Other studies (Sobotka & Jurčák 2009;Watanabe et al 2009b) do not show significantly lower field strengths in UDs. While upwellings in the deep layers of UDs were found (Socas-Navarro et al 2004;Rimmele 2004;Riethmüller et al 2008a;Bharti et al 2007aBharti et al , 2009, the jet-like upflows predicted by the simulations in the higher layers have not been observed so far. This may be due to the fact that spectroscopic studies mostly refer to large UDs, for which the jets are weaker or absent.…”

Section: Ud Properties and Optical-depth Dependence At 630 Nmmentioning

confidence: 65%

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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Section: Ud Properties and Optical-depth Dependence At 630 Nmmentioning

confidence: 65%

Properties of simulated sunspot umbral dots

Bharti¹,

Beeck²,

Schüßler³

2010

A&A

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“…These show the formation of umbral dots, including the dark cores crossing them that have been observed in large dots (for recent references see Bharti et al 2007;Rimmele 2008;Riethmüller et al 2008).…”

Section: Observational Connectionsmentioning

confidence: 93%

Striation and convection in penumbral filaments

Spruit¹,

Scharmer

Löfdahl

2010

A&A

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Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and "canals". In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.

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“…The analysis of high resolution Doppergrams and continuum images of sunspot umbrae by Bharti et al (2007a) show that the umbral dots are often situated adjacent to downwflows. Localized downflows near UDs have also been observed by Ortiz et al (2010).…”

Section: Comparison With Observationsmentioning

confidence: 99%

Structures in compressible magnetoconvection and the nature of umbral dots

Tian

Petrovay

2013

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Context. Structures seen in idealized numerical experiments on compressible magnetoconvection in an imposed strong vertical magnetic field show important differences from those detected in observations or realistic numerical simulations of sunspot umbrae. Aims. To elucidate the origin of these discrepancies, we present a series of idealized 3D compressible magnetoconvection experiments that differ from previous such experiments in several details, bringing them closer to realistic solar conditions. Methods. An initially vertical magnetic field B 0 is imposed on a time snapshot of fully developed solar-like turbulent convection in a layer bounded by a stable layer from above. Upon relaxation to a statistically steady state, the structure of the flow field and magnetic field is examined. Results. Instead of the vigorous granular convection (GRC) well known to take place in magnetized or weakly magnetized convection, for high values of B 0 heat is transported by small-scale convection (SSC) in the form of narrow, persistent convective columns consisting of slender upflows accompanied by adjacent downflow patches, which are reminiscent of the "convectons" identified in earlier semianalytic models. For moderate field strengths, flux separation (FXS) is observed: isolated field-free inclusions of GRC are embedded in a strongly magnetized plasma with SSC. Between the SSC and FXS regimes, a transitional regime (F/S) is identified where convectons dynamically evolve into multiply segmented granular inclusions and back. Conclusions. Our results agree in some aspects more closely with observed umbral structures than earlier idealized models, because they do reproduce the strong localized, patchy downflows immediately adjacent to the narrow convective columns. Based on recent observations of umbral dots, we suggest that in some cases the conditions in sunspot umbrae correspond to the newly identified F/S transitional regime.

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Evidence for Magnetoconvection in Sunspot Umbral Dots

Cited by 39 publications

References 22 publications

Properties of simulated sunspot umbral dots

Properties of simulated sunspot umbral dots

Striation and convection in penumbral filaments

Structures in compressible magnetoconvection and the nature of umbral dots

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