Evolution and wave-like properties of the average solar supergranule

Langfellner, Jan; Birch, A. C.; Gizon, Laurent

doi:10.1051/0004-6361/201732471

Cited by 21 publications

(23 citation statements)

References 42 publications

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“…North-South alignments of supergranules have also been found in the polar regions by Nagashima et al (2011) using local helioseismology with Hinode/SOT data. A recent study by Langfellner et al (2018) using both time-distance helioseismology on SDO/HMI data and correlation tracking appears to further confirm the results of on the wave-like oscillatory properties of the dynamics in the range of scales 50 < < 120 comparable to or slightly larger than the supergranulation scale.…”

Section: Effects Of Rotationsupporting

confidence: 66%

The Sun’s supergranulation

Rincon

Rieutord

2018

Living Rev Sol Phys

110

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Supergranulation is a fluid-dynamical phenomenon taking place in the solar photosphere, primarily detected in the form of a vigorous cellular flow pattern with a typical horizontal scale of approximately 30-35 megameters, a dynamical evolution time of 24-48 h, a strong 300-400 m/s (rms) horizontal flow component and a much weaker 20-30 m/s vertical component. Supergranulation was discovered more than sixty years ago, however, explaining its physical origin and most important observational characteristics has proven extremely challenging ever since, as a result of the intrinsic multiscale, nonlinear dynamical complexity of the problem concurring with strong observational and computational limitations. Key progress on this problem is now taking place with the advent of 21st-century supercomputing resources and the availability of global observations of the dynamics of the solar surface with high spatial and temporal resolutions. This article provides an exhaustive review of observational, numerical and theoretical research on supergranulation, and discusses the current status of our understanding of its origin and dynamics, most importantly in terms of large-scale nonlinear thermal convection, in the light of a selection of recent findings. Keywords Supergranulation · Convection · Turbulence · MHD 4 François Rincon, Michel Rieutord 6 François Rincon, Michel Rieutord

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Section: Effects Of Rotationsupporting

confidence: 66%

The Sun’s supergranulation

Rincon

Rieutord

2018

Living Rev Sol Phys

110

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“…For example, it may be possible to produce models in which the emergence locations are in some way related to supergranulation-scale diverging flows rather than converging flows. The connection between the locations of supergranulation centers and supergranulation-scale convergence regions is only statistical in nature ( Langfellner et al 2018 ), so that there is a distinction between models based on convergence centers and supergranule centers. We also note that the model presented here is the simplest model that we found that provides a qualitative match to the observations.…”

Section: Discussionmentioning

confidence: 99%

Average surface flows before the formation of solar active regions and their relationship to the supergranulation pattern

Birch

Schunker

Braun

et al. 2019

A&A

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Context. The emergence of solar active regions is an important but poorly understood aspect of the solar dynamo. Aims. Knowledge of the flows associated with the rise of active-region-forming magnetic concentrations through the near-surface layers will help determine the mechanisms of active region formation. Methods. We used helioseismic holography and granulation tracking to measure the horizontal flows at the surface that precede the emergence of active regions. We then averaged these flows over about sixty emerging active regions to reduce the noise, selecting active regions that emerge into relatively quiet Sun. To help interpret the results, we constructed a simple model flow field by generating synthetic “emergence locations” that are probabilistically related to the locations of supergranulation-scale convergence regions in the quiet Sun. Results. The flow maps obtained from helioseismology and granulation tracking are very similar (correlation coefficients for single maps around 0.96). We find that active region emergence is, on average, preceded by converging horizontal flows of amplitude about 40 m s−1. The convergence region extends over about 40 Mm in the east-west direction and about 20 Mm in the north-south direction and is centered in the retrograde direction relative to the emergence location. This flow pattern is largely reproduced by a model in which active region emergence occurs preferentially in the prograde direction relative to supergranulation inflows. Conclusions. Averaging over many active regions reveals a statistically significant pattern of near-surface flows prior to emergence. The qualitative success of our simple model suggests that rising flux concentrations and supergranule-scale flows interact during the emergence process.

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“…We use maps of the horizontal near-surface divergence that were obtained by Langfellner et al (2018) using Dopplergrams and intensity images from HMI/SDO. The data cover 365 days from May 2010 through April 2011.…”

Section: Divergence Maps Of Supergranular Flowsmentioning

confidence: 99%

“…Using the intensity images and LCT of granules (November & Simon 1988), Langfellner et al (2018) obtained estimates of the two horizontal flow components for the same spatial and temporal coverage. This results in maps with 97×97 pixels, each of size (5 × 0.348 Mm) 2 .…”

Section: Divergence Maps Of Supergranular Flowsmentioning

confidence: 99%

“…In this work, we apply bispectral analysis to near-surface horizontal divergence maps of supergranulation obtained by Langfellner et al (2018) using local correlation tracking (LCT, November & Simon 1988) and time-distance helioseismology (TD, Duvall et al 1993) and data from the Helioseismic and Magnatic Imager (HMI, Scherrer et al 2012;Schou et al 2012) onboard the Solar Dynamics Observatory (SDO, Pesnell et al 2012).…”

Section: Introductionmentioning

confidence: 99%

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Characterizing the spatial pattern of solar supergranulation using the bispectrum

Böning

Birch

Gizon

et al. 2020

A&A

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Context. The spatial power spectrum of supergranulation does not fully characterize the underlying physics of turbulent convection. For example, it does not describe the non-Gaussianity in the horizontal flow divergence. Aims. Our aim is to statistically characterize the spatial pattern of solar supergranulation beyond the power spectrum. The next-order statistic is the bispectrum. It measures correlations of three Fourier components and is related to the nonlinearities in the underlying physics. It also characterizes how a skewness in the dataset is generated by the coupling of three Fourier components. Methods. We estimated the bispectrum of supergranular horizontal surface divergence maps that were obtained using local correlation tracking (LCT) and time-distance helioseismology (TD) from one year of data from the Helioseismic and Magnetic Imager on-board the Solar Dynamics Observatory starting in May 2010. Results. We find significantly nonzero and consistent estimates for the bispectrum using LCT and TD. The strongest nonlinearity is present when the three coupling wave vectors are at the supergranular scale. These are the same wave vectors that are present in regular hexagons, which have been used in analytical studies of solar convection. At these Fourier components, the bispectrum is positive, consistent with the positive skewness in the data and consistent with supergranules preferentially consisting of outflows surrounded by a network of inflows. We use the bispectral estimates to generate synthetic divergence maps that are very similar to the data. This is done by a model that consists of a Gaussian term and a weaker quadratic nonlinear component. Using this method, we estimate the fraction of the variance in the divergence maps from the nonlinear component to be of the order of 4-6%.Conclusions. We propose that bispectral analysis is useful for understanding the dynamics of solar turbulent convection, for example for comparing observations and numerical models of supergranular flows. This analysis may also be useful to generate synthetic flow fields.

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Evolution and wave-like properties of the average solar supergranule

Cited by 21 publications

References 42 publications

The Sun’s supergranulation

The Sun’s supergranulation

Average surface flows before the formation of solar active regions and their relationship to the supergranulation pattern

Characterizing the spatial pattern of solar supergranulation using the bispectrum

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