The formation of a sunspot penumbra

Schlichenmaier, R.; Rezaei, Reza; González, N. Bello; Waldmann, T.

doi:10.1051/0004-6361/201014112

Cited by 81 publications

(83 citation statements)

References 17 publications

(16 reference statements)

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“…The values are 171.4 and 104.8 Mm 2 per day, respectively. This is about four times faster than the corresponding decay times but less than one half of the penumbral growth rate of about 400 Mm 2 per day presented by Schlichenmaier et al (2010).…”

Section: Photospheric Evolutionmentioning

confidence: 56%

“…The formation of a penumbra around a sunspot occurs rapidly, i.e., within a few hours a sunspot can develop a penumbra (Leka & Skumanich 1998;Yang et al 2003), and this rapidity is intimately linked to the more inclined magnetic field lines and the onset of the Evershed flow. Schlichenmaier et al (2010) observed the growth of a penumbra where only the newly formed penumbra contributed to the increase in spot size while the umbra remained stable. The formation of a penumbra, which would surround the entire spot, was hindered by continuous flux emergence between the spots of the bipolar region.…”

Section: Introductionmentioning

confidence: 99%

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Horizontal flow fields observed in Hinode G-band images

Verma

Balthasar

Deng

et al. 2012

A&A

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Context. Generation and dissipation of magnetic fields is a fundamental physical process on the Sun. In comparison to flux emergence and the initial stages of sunspot formation, the demise of sunspots still lacks a comprehensive description. Aims. The evolution of sunspots is most commonly discussed in terms of their intensity and magnetic field. Here, we present additional information about the three-dimensional flow field in the vicinity of sunspots towards the end of their existence. Methods. We present a subset of multi-wavelengths observations obtained with the Japanese Hinode mission, the Solar Dynamics Observatory (SDO), and the Vacuum Tower Telescope (VTT) at Observatorio del Teide, Tenerife, Spain during the time period 2010 November 18−23. Horizontal proper motions were derived from G-band and Ca ii H images, whereas line-of-sight velocities were extracted from VTT echelle Hα λ656.28 nm spectra and Fe i λ630.25 nm spectral data of the Hinode/Spectro-Polarimeter, which also provided three-dimensional magnetic field information. The Helioseismic and Magnetic Imager on board SDO provided continuum images and line-of-sight magnetograms, in addition to the high-resolution observations for the entire disk passage of the active region. Results. We perform a quantitative study of photospheric and chromospheric flow fields in and around decaying sunspots. In one of the trailing sunspots of active region NOAA 11126, we observe moat flow and moving magnetic features (MMFs), even after its penumbra had decayed. We also detect a superpenumbral structure around this pore. We find that MMFs follow well-defined, radial paths from the spot all the way to the border of a supergranular cell surrounding the spot. In contrast, flux emergence near the other sunspot prevents the establishment of similar well ordered flow patterns, which could be discerned around a tiny pore of merely 2 Mm diameter. After the disappearance of the sunspots/pores, a coherent patch of abnormal granulation remained at their location, which was characterized by more uniform horizontal proper motions, low divergence values, and smaller photospheric Doppler velocities. This region, thus, differs significantly from granulation and other areas covered by G-band bright points. We conclude that this peculiar flow pattern is a signature of sunspot decay and the dispersal of magnetic flux.

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Section: Photospheric Evolutionmentioning

confidence: 56%

Section: Introductionmentioning

confidence: 99%

Horizontal flow fields observed in Hinode G-band images

Verma

Balthasar

Deng

et al. 2012

A&A

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“…2 are very similar in the Ca ii K and broadband images. These motions can also be discerned in the G-band and Ca ii K animations by Schlichenmaier et al (2010a).…”

Section: Shear Motions Along a Light Bridgementioning

confidence: 84%

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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“…As pointed out by Baumann & Solanki (2005), it is unlikely that sunspots are entirely the result of such fragmentation. It is perhaps more reasonable to imagine that they are the joint product of fragmentation and coalescence of flux patches, as discussed by Zwaan (1992) and Schlichenmaier et al (2010b).…”

Section: Introductionmentioning

confidence: 99%

Properties of sunspot umbrae observed in cycle 24

Kiess

Rezaei

Schmidt

2014

A&A

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Aims. There is an ongoing debate whether the solar activity cycle is overlaid with a long-term decline that may lead to another grand minimum in the near future. We used the size, intensity, and magnetic field strength of sunspot umbrae to compare the present cycle 24 with the previous one. Methods. We used data of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and selected all sunspots between May 2010 and October 2012, using one image per day. We created two subsets of this dataset with a manual tracking algorithm, both without duplication. One contains each sunspot (910 umbrae within 488 spots) and was used to analyze the distribution of umbral areas, selected with an automated thresholding method. The other subset contains 205 fully evolved sunspots. We estimated their magnetic field and the total magnetic flux and discuss the relations between umbral size, minimum continuum intensity, maximum field strength, and total magnetic flux. Results. We find non-linear relations between umbral minimum intensity and size and between maximum magnetic field strength and size. The field strength scales linearly with the intensity and the umbral size scales roughly linearly with the total magnetic flux, while the size and field strength level off with stronger flux. When separated into hemispheres and averaged temporally, the southern umbrae show a temporal increase in size and the northern umbrae remain constant. We detected no temporal variation in the umbral mean intensity. The probability density function of the umbral area in the ascending phase of the current solar cycle is similar to that of the last solar cycle. Conclusions. From our investigation of umbral area, magnetic field, magnetic flux, and umbral intensity of the sunspots of the rising phase of cycle 24, we do not find a significant difference to the previous cycle, and hence no indication for a long-term decline of solar activity.

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The formation of a sunspot penumbra

Cited by 81 publications

References 17 publications

Horizontal flow fields observed in Hinode G-band images

Horizontal flow fields observed in Hinode G-band images

Shear and vortex motions in a forming sunspot

Properties of sunspot umbrae observed in cycle 24

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