Properties of sunspot moats derived from horizontal motions

Sobotka, M.; Roudier, Th.

doi:10.1051/0004-6361:20077552

Cited by 32 publications

(59 citation statements)

References 22 publications

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“…During the initiation of the MF, the magnetic components in the vicinity of the sunspot are pushed to the periphery and leave the moat cell largely nonmagnetic with single MMFs. The extension of the moat field reaches between 10 Mm to 20 Mm from the penumbral boundary for small sunspots and can be roughly twice the spot radius for larger spots (Brickhouse & Labonte 1988), but Sobotka & Roudier (2007) find no such correspondence.…”

Section: Introductionmentioning

confidence: 79%

“…The MF is present as a stationary flow through the spot's lifetime, which varies between several weeks to several months (see, e.g., Solanki 2003): Sobotka & Roudier (2007) do not find any significant variations in the MF within half a day. During the initiation of the MF, the magnetic components in the vicinity of the sunspot are pushed to the periphery and leave the moat cell largely nonmagnetic with single MMFs.…”

Section: Introductionmentioning

confidence: 99%

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Correlations between sunspots and their moat flows

Löhner-Böttcher

Schlichenmaier

2013

A&A

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Context. The presence of the moat flow around sunspots is intimately linked to the mere existence of sunspots. Aims. We characterize the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our knowledge of sunspot structures and photospheric flow properties. Methods. We calibrated HMI synoptic Doppler maps and used them to analyze 3 h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50 • . Assuming axially symmetrical flow fields, we infer the azimuthally averaged horizontal velocity component of the MF and EF from 51 velocity maps. We studied the MF properties (velocity and extension) and elaborate on how these components depend on sunspot parameters (sunspot size and EF velocity). To explore the weekly and monthly evolution of MFs, we compare spots rotating from the eastern to western limbs and spots that reappear on the eastern limb. Results. Our calibration procedure of HMI Doppler maps yields reliable and consistent results. In 3 h averages, we find the MF decreases on average from some 1000 ± 200 m/s just outside the spot boundary to 500 m/s after an additional 4 Mm. The average MF extension lies at 9.2 ± 5 Mm, where the velocity drops below some 180 m/s. Neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity. But, the EF velocity does show a tendency to be enhanced with sunspot size. On a time scale of a week and a month, we find decreasing MF extensions and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF. Conclusions. On 3 h averages, the EF velocity scales with the size of sunspots, while the MF properties show no significant correlation with the EF or with the sunspot size. This we interpret as a hint that the physical origins of EF and MF are distinct.

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Section: Introductionmentioning

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Correlations between sunspots and their moat flows

Löhner-Böttcher

Schlichenmaier

2013

A&A

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“…3.5, we discuss some properties of the moat flow based on the proper motions of MMFs. Sobotka & Roudier (2007) determined that the moat radius is independent of the spot size. However, Balthasar & Muglach (2010) found that the moat flow terminates at a distance of four times the spot radius -in contrast to three times the spot radius in the present study.…”

Section: Horizontal Proper Motionsmentioning

confidence: 99%

Horizontal flow fields observed in Hinode G-band images

Verma

Balthasar

Deng

et al. 2012

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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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“…It is tempting to relate the distinct east-west asymmetry of the bright halo depicted in Fig. 7 to that of the moat around sunspots (Sobotka & Roudier 2007). However, this lopsidedness is neither found in intensity nor in the velocity and divergence maps (Fig.…”

Section: Discussionmentioning

confidence: 92%

Horizontal flow fields observed in Hinode G-band images

Verma

Denker

2014

A&A

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Context. Solar pores are penumbra-lacking magnetic features, that mark two important transitions in the spectrum of magnetohydrodynamic processes: (1) the magnetic field becomes sufficiently strong to suppress the convective energy transport and (2) at some critical point some pores develop a penumbra and become sunspots. Aims. The purpose of this statistical study is to comprehensively describe solar pores in terms of their size, perimeter, shape, photometric properties, and horizontal proper motions. The seeing-free and uniform data of the Japanese Hinode mission provide an opportunity to compare flow fields in the vicinity of pores in different environments and at various stages of their evolution. Methods. The extensive database of high-resolution G-band images observed with the Hinode Solar Optical Telescope (SOT) is a unique resource to derive statistical properties of pores using advanced digital image processing techniques. The study is based on two data sets: (1) , when the pores migrate from high to low latitudes. Pores are rarely encountered in quiet-Sun G-band images, and only about 10% of pores exist in isolation. In general, pores do not exhibit a circular shape. Typical aspect ratios of the semi-major and -minor axes are 3:2 when ellipses are fitted to pores. Smaller pores (more than two-thirds are smaller than 5 Mm 2 ) tend to be more circular, and their boundaries are less corrugated. Both the area and perimeter length of pores obey log-normal frequency distributions. The frequency distribution of the intensity can be reproduced by two Gaussians representing dark and bright components. Bright features resembling umbral dots and even light bridges cover about 20% of the pores' area. Averaged radial profiles show a peak in the intensity at normalized radius R N = r/R pore = 2.1, followed by maxima of the divergence at R N = 2.3 and the radial component of the horizontal velocity at R N = 4.6. The divergence is negative within pores strongly suggesting converging flows towards the center of pores, whereas exterior flows are directed towards neighboring supergranular boundaries. The photometric radius of pores, where the intensity reaches quiet-Sun levels at R N = 1.4, corresponds to the position where the divergence is zero at R N = 1.6. Conclusions. Morphological and photometric properties as well as horizontal flow fields have been obtained for a statistically meaningful sample of pores. This provides critical boundary conditions for MHD simulations of magnetic flux concentrations, which eventually evolve into sunspots or just simply erode and fade away. Numerical models of pores (and sunspots) have to fit within these confines, and more importantly ensembles of pores have to agree with the frequency distributions of observed parameters.

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Properties of sunspot moats derived from horizontal motions

Cited by 32 publications

References 22 publications

Correlations between sunspots and their moat flows

Correlations between sunspots and their moat flows

Horizontal flow fields observed in Hinode G-band images

Horizontal flow fields observed in Hinode G-band images

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