Observations of Dark Lanes in Umbral Fine Structure from the<i>Hinode</i>Solar Optical Telescope: Evidence for Magnetoconvection

Bharti, L.; Joshi, C.; Jaaffrey, S. N. A.

doi:10.1086/523352

Cited by 41 publications

(46 citation statements)

References 19 publications

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“…Dark lanes in UDs, predicted in the simulations by Schüssler & Vögler (2006), have been observed by Bharti et al (2007b) in large (>1 ) bright UD-like features seen in Hinode/SOT G-band images with diffraction-limited spatial resolution of 0. 22.…”

Section: Introductionmentioning

confidence: 56%

Morphology and evolution of umbral dots and their substructures

Sobotka¹,

Puschmann²

2009

A&A

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Context. Substructures -dark lanes and tails -of umbral dots (UDs) were predicted by numerical simulations of magnetoconvection and have been detected later in some observations. Aims. To provide constraints for realistic theoretical models of sunspot umbrae, we describe the observed properties and evolutionary characteristics of UDs (including their substructure) and of other umbral structures. Methods. We analyse a 6 h 23 min time series of broadband images of a large umbra in the active region NOAA 10634, acquired with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A 43 min part of this series was reconstructed with the MFBD method, reaching a spatial resolution of 0. 14. With the help of image segmentation, feature tracking, and local correlation tracking, we measured brightness, size, lifetime, and horizontal velocities of various umbral structures. Results. Large structures in the umbra -strong and faint light bridges (LBs) and an extended penumbral filament -evolve on time scales of hours. Most (90%) of UDs and bright point-like features in faint LBs split and merge, and their median lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is considered as the end of their life. Both UDs and features in faint LBs that do not split or merge are clearly smaller (0. 15) than the average size (0. 17) of all features. Horizontal motions of umbral bright small-scale features are directed either into the umbra or along faint LBs with mean horizontal velocities of 0.34 km s −1 . Features faster than 0.4 km s −1 appear mostly at the periphery of the umbra. The motion of peripheral UDs (PUDs) seems to be the continuation of the motion of penumbral grains (PGs). The intensity of dark lanes, measured in four bright central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs. The width of dark lanes is probably less than the resolution limit 0. 14. The characteristic time of substructure changes of UDs is ∼4 min. We observe narrow (0. 14) bright and dark filaments connected with PUDs. The bright filaments are 0.06 I ph brighter than the dark ones. Usually one dark and two bright filaments form a 0. 4 wide tail attached to one PUD, resembling a short dark-cored penumbral filament. Conclusions. Our results indicate the similarity between PUDs and PGs located at the tips of bright penumbral filaments. The features seen in numerical MHD simulations are consistent with our observations of dark lanes in CUDs and tails attached to PUDs.

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Section: Introductionmentioning

confidence: 56%

Morphology and evolution of umbral dots and their substructures

Sobotka¹,

Puschmann²

2009

A&A

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“…These UDs rarely have the regular "coffee bean" structure suggested by the simulations of Rempel et al (2009b); instead, dark lanes are often asymmetrically positioned inside them, and multiple dark lanes are common, lending a segmented appearance to these UDs. Umbral dots with up to six dark lanes have been observed by Bharti et al (2007b). Observations also indicate that the UDs evolve rapidly compared to sunspots.…”

Section: Comparison With Observationsmentioning

confidence: 86%

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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“…Narrow nearly field-free upflowing umbral dots adjacent to downflows can be the result of convection in a strong magnetic field (Weiss et al, 1990). Recent observations seem to confirm this idea (see, e.g., Bharti et al, 2007;Watanabe et al, 2009Watanabe et al, , 2012. In particular, there are measurements suggesting that the upflowing mass flux of umbral dots and the downflowing flux in their surroundings can be balanced, which is characteristic for magnetoconvection (Riethmüller et al, 2013).…”

Section: Magneto-acoustic Wavesmentioning

confidence: 86%

Oscillations and Waves in Sunspots

Khomenko

Collados

2015

Living Rev. Sol. Phys.

157

118

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A magnetic field modifies the properties of waves in a complex way. Significant advances have been made recently in our understanding of the physics of sunspot waves with the help of high-resolution observations, analytical theories, as well as numerical simulations. We review the current ideas in the field, providing the most coherent picture of sunspot oscillations as by present understanding. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

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Observations of Dark Lanes in Umbral Fine Structure from theHinodeSolar Optical Telescope: Evidence for Magnetoconvection

Cited by 41 publications

References 19 publications

Morphology and evolution of umbral dots and their substructures

Morphology and evolution of umbral dots and their substructures

Structures in compressible magnetoconvection and the nature of umbral dots

Oscillations and Waves in Sunspots

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