Evidence of convective rolls in a sunspot penumbra

Zakharov, V.; Hirzberger, J.; Riethmüller, T. L.; Solanki, S. K.; Kobel, Philippe

doi:10.1051/0004-6361:200810266

Cited by 62 publications

(91 citation statements)

References 21 publications

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“…These downflows have yet to be confirmed by observations. Zakharov et al (2008) found a weak downflow on the center-ward side of a filament structure where the upflow was observed. However, the Milne-Eddington inversion used placed the strongest and most vertical magnetic field in the location of the downflow, which is inconsistent with the proposed overturning convection.…”

Section: Introductionmentioning

confidence: 94%

Temporal downflows in a penumbra

Jurčák

Katsukawa

2010

A&A

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Aims. We analyze temporal downflow patches that are located in a penumbra and have the same polarity of the magnetic field as a sunspot umbra. Methods. The repetitive 2 wide raster scans of penumbral regions that are taken with one minute cadence by the Hinode spectropolarimeter are used to detect the line-of-sight velocities in the penumbra from enhanced signals in the wings of Stokes V profiles. The lifetimes and positions within penumbra of the identified downflow patches are investigated. The plasma properties of the downflow patches are determined using the inversions of the observed Stokes profiles. Results. The temporal downflows have lifetimes of up to fourteen minutes. Some of them are related to the disappearance or weakening of nearby upflow regions or to the chromospheric brightenings. The downflows take place in regions with stronger and more vertical magnetic fields than the upflow regions.

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Section: Introductionmentioning

confidence: 94%

Temporal downflows in a penumbra

Jurčák

Katsukawa

2010

A&A

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“…This is inevitably a A25, page 11 of 17 very crude estimate at best, since the SPINOR inversion code calculates the density using hydrostatic equilibrium, an approximation that is not valid in a strongly magnetized atmosphere. In addition, since the vertical coordinate relevant for the inversions is not a geometric coordinate, but indicates constant optical depth, the line-of-sight velocity is not necessarily normal to the surface over which we calculate the flux, because of the corrugated shape of the penumbral filaments, as inferred by Zakharov et al (2008). In particular, since the opacity is a strong function of the temperature, the optical depth surfaces will be inclined towards higher layers in the direction of any temperature gradient.…”

Section: Mass Fluxmentioning

confidence: 99%

Structure of sunspot penumbral filaments: a remarkable uniformity of properties

Tiwari

Noort

Lagg

et al. 2013

A&A

205

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Context. The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear. Aims. We aim to investigate the fine-scale structure of these penumbral filaments. Methods. We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope.Results. Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800 K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1 kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth. Conclusions. We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature.

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“…These authors did not give a clear interpretation, but nevertheless suggested that it is due to "twisting motions" of the magnetic field, a view that also dominates later references. A more explicit interpretation was made by Zakharov et al (2008) using observations with the Swedish 1-m Solar Telescope (SST). They show that a twisting field explanation conflicts directly with the observed motion of the pattern, which is systematically away from the solar limb.…”

Section: Observational Connectionsmentioning

confidence: 99%

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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Evidence of convective rolls in a sunspot penumbra

Cited by 62 publications

References 21 publications

Temporal downflows in a penumbra

Temporal downflows in a penumbra

Structure of sunspot penumbral filaments: a remarkable uniformity of properties

Striation and convection in penumbral filaments

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