Normal and counter Evershed flows in the photospheric penumbra of a sunspot

Siu-Tapia, A.; Lagg, A.; Solanki, S. K.; Noort, M. van; Jurčák, J.

doi:10.1051/0004-6361/201730647

Cited by 25 publications

(65 citation statements)

References 47 publications

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“…In contrast, the superstrong fields reported by Siu-Tapia et al (2017) (> 7 kG based on SPINOR 2D inversions) were observed in the tails of inverted penumbral filaments carrying a counter-EF (CEF), i.e. a gas inflow towards the sunspot umbra at photospheric heights, near the inner penumbral boundary.…”

Section: Introductionmentioning

confidence: 89%

“…For the SPINOR 2D inversions used in this work (cf. Siu-Tapia et al 2017) all the atmospheric free parameters were initially defined at three optical depth nodes, placed at log(τ) = −2, −0.8, and 0. This set of optical depth nodes was shown to work well for the inversion of Hinode/SP observations of sunspots by Siu-Tapia et al (2017).…”

Section: Observations and Spatially Coupled Inversionsmentioning

confidence: 99%

“…Siu-Tapia et al 2017) all the atmospheric free parameters were initially defined at three optical depth nodes, placed at log(τ) = −2, −0.8, and 0. This set of optical depth nodes was shown to work well for the inversion of Hinode/SP observations of sunspots by Siu-Tapia et al (2017). At each of the three chosen nodes the temperature T , magnetic field strength B, field inclination relative to the line-of-sight γ LOS , field azimuth φ, line-of-sight velocity v LOS , and a microturbulent velocity v MIC , were fitted, leading to 18 free parameters in total.…”

Section: Observations and Spatially Coupled Inversionsmentioning

confidence: 99%

“…Hence, the reliability of the inversion results for such peculiar pixels needs to be tested. This work is aimed at discussing the reliability of those penumbral fields in excess of 7 kG reported by Siu-Tapia et al (2017) concentrated at the inner boundary of the penumbra of the main sunspot in active region (AR) 10930. Here we examine the reality of such findings by applying some classical diagnostic methods and various inversion techniques with different model atmospheres to the Stokes profiles observed with the spectropolarimeter (SP) of the Solar Optical Telescope (SOT) on board Hinode.…”

Section: Introductionmentioning

confidence: 99%

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Superstrong photospheric magnetic fields in sunspot penumbrae

Siu-Tapia

Lagg

Noort

et al. 2019

A&A

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Context. Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed. Aims. Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930. Methods. We apply classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compare the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at τ = 1) associated with supersonic downflows. Results. The different inversions provide different results: while the SPINOR 2D inversions consider a height-dependent singlecomponent model and return B>7 kG and supersonic positive v LOS (corresponding to a counter-Evershed flow), height-dependent 2-component inversions suggest the presence of an umbral component (almost at rest) with field strengths ∼ 4 − 4.2 kG and a penumbral component with v LOS ∼ 16 − 18 km s −1 and field strengths up to ∼ 5.8 kG. Likewise, height-independent 2-component inversions find a solution for an umbral component and a strongly redshifted (v LOS ∼ 15 − 17 km s −1 ) penumbral component with B∼ 4 kG.According to a Bayesian Information Criterion, the inversions providing a better balance between the quality of the fits and the number of free parameters considered by the models are the height -independent 2-component inversions, but they lie only slightly above the SPINOR 2D inversions. Since it is expected that the physical parameters all display considerable gradients with height, as supported by MHD sunspot simulations, the SPINOR 2D inversions are the preferred ones. Conclusions. According to the MHD sunspot simulation analyzed here, the presence of counter-Evershed flows in the photospheric penumbra can lead to the necessary conditions for the observation of ∼ 5 kG fields at the inner penumbra. Although a definite conclusion about the potential existence of fields in excess of 7 kG cannot be given, their nature could be explained (based on the simulation results) such as the consequence of the extreme dynamical effects introduced by highly supersonic counter-Evershed flows (v LOS > 10 km s −1 and up to ∼30 km s −1 according to SPINOR 2D), which would be much faster and more compressive downflows than those found in the MHD simulations and therefore could lead to the field intensification up to considerably stronger fields. Also, a lower gas density would lead to a deeper depression of the τ = 1 surfa...

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Section: Introductionmentioning

confidence: 89%

Section: Observations and Spatially Coupled Inversionsmentioning

confidence: 99%

Section: Observations and Spatially Coupled Inversionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Superstrong photospheric magnetic fields in sunspot penumbrae

Siu-Tapia

Lagg

Noort

et al. 2019

A&A

Self Cite

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“…Counter Evershed flows were also found in umbral filaments (e.g., Kleint & Sainz Dalda 2013), during the formation of penumbrae (Schlichenmaier et al 2012;Romano et al 2014;Murabito et al 2016Murabito et al , 2018, and over stable penumbrae (Louis et al 2014;Siu-Tapia et al 2017). Lim et al (2011) presented observations of two granular-like features (GLFs), about 1 ′′ wide, at the tip of penumbral filaments and associated with MMFs.…”

Section: Discussionmentioning

confidence: 99%

Penumbral Brightening Events Observed in AR NOAA 12546

Murabito

Guglielmino

Ermolli

et al. 2020

ApJ

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Penumbral transient brightening events have been attributed to magnetic reconnection episodes occurring in the low corona. We investigated the trigger mechanism of these events in active region NOAA 12546 by using multi-wavelength observations obtained with the Interferometric Bidimensional Spectrometer (IBIS), by the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS), and the Hinode satellites. We focused on the evolution of an area of the penumbra adjacent to two small-scale emerging flux regions (EFRs), which manifested three brightening events detected from the chromosphere to the corona. Two of these events correspond to B-class flares. The same region showed short-lived moving magnetic features (MMFs) that streamed out from the penumbra. In the photosphere, the EFRs led to small-scale penumbral changes associated with a counter-Evershed flow and to a reconfiguration of the magnetic fields in the moat. The brightening events had one of the footpoints embedded in the penumbra and seemed to result from the distinctive interplay between the pre-existing penumbral fields, MMFs, and the EFRs. The IRIS spectra measured therein reveal enhanced temperature and asymmetries in spectral lines, suggestive of event triggering at different height in the atmosphere. Specifically, the blue asymmetry noted in C II and Mg II h&k lines suggests the occurrence of chromospheric evaporation at the footpoint located in the penumbra as a consequence of magnetic reconnection process at higher atmospheric heigths.

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