The Stellar Dynamo

Nesme-Ribes, E.; Baliunas, S. L.; Sokoloff, Dmitry

doi:10.1038/scientificamerican0896-46

Cited by 25 publications

(25 citation statements)

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“…This range in d1 agrees well with measurements by Nesme-Ribes et al (1996), but not with those reported by Meunier (1999Meunier ( , 2004, who found generally a higher minimum value (around 1.4). From the analysis of both real and simulated data (Sect.…”

Section: Fractal Dimension and Structure Sizesupporting

confidence: 81%

“…5.3), as the full-disk MDI data she analyzed are unaffected by atmospheric degradation. The plateau in d1 beyond object sizes of 2000 Mm 2 also agrees with previous results, but this time more so with those of Meunier (2004) and less so with Nesme-Ribes et al (1996), who found the plateau to occur already for structures of size >300 square-pixels (corresponding to about 500 Mm 2 ). Tests with both real and simulated data suggest that this difference may lie in the area range over which the fit for d1 at each point was made, a value not quoted by NesmeRibes et al (1996), but taken in our study to be ∆ log A = 1.5 in agreement with that used by Meunier (2004).…”

Section: Fractal Dimension and Structure Sizesupporting

confidence: 80%

“…These preselected regions are made up of those pixels which are brighter than a fixed contrast value on a spatially filtered image (box average side = R sun 20 ). A second contrast threshold value, determined by the procedure explained in Nesme-Ribes et al (1996), is then used to single out from the pre-selected regions all those features identified for our study. Pixels representative of sunspots and pores, previously identified from red continuum images, were excluded from the identified features.…”

Section: Feature Identificationmentioning

confidence: 99%

“…In the first, we performed a simple linear fit to the logarithm of the perimeters and areas measured for structures of different sizes; we indicate the fractal dimension so obtained as D. In order to investigate the size dependence of the fractal dimension estimated in this way, the fit is performed over the entire data set or over objects in a specified size range. In the second, we adopted a method first proposed by Nesme-Ribes et al (1996) and later employed by Meunier (1999Meunier ( , 2004, in which perimeter and area values are averaged over bins in area, each of width ∆ log A = 0.05, and the fitting is done on these averages for a series of overlapping windows of constant width ∆ log A = 1.5, producing a measure of d which is a function of A. We indicate the fractal dimension estimated in this way as d1.…”

Section: Fractal Dimension Definitionmentioning

confidence: 99%

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On the reliability of the fractal dimension measure of solar magnetic features and on its variation with solar activity

Criscuoli

Rast

Ermolli³

et al. 2006

A&A

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Context. Several studies have investigated the fractal and multifractal nature of magnetic features in the solar photosphere and its variation with the solar magnetic activity cycle. Aims. Here we extend those studies by examining the fractal geometry of bright magnetic features at higher atmospheric levels, specifically in the solar chromosphere. We analyze structures identified in CaIIK images obtained with the Precision Solar Photometric Telescopes (PSPTs) at Osservatorio Astronomico di Roma (OAR) and Mauna Loa Solar Observatory (MLSO). Methods. Fractal dimension estimates depend on the estimator employed, the quality of the images, and the structure identification techniques used. We examine both real and simulated data and employ two different perimeter-area estimators in order to understand the sensitivity of the deduced fractal properties to pixelization and image quality. Results. The fractal dimension of bright "magnetic" features in CaIIK images ranges between values of 1.2 and 1.7 for small and large structures respectively. This size dependency largely reflects the importance of image pixelization in the measurement of small objects. The fractal dimension of chromospheric features does not show any clear systematic variation with time over the period examined, the descending phase of solar cycle 23. Conclusions. These conclusions, and the analysis of both real and synthetic images on which they are based, are important in the interpretation of previously reported results.

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Section: Fractal Dimension and Structure Sizesupporting

confidence: 81%

Section: Fractal Dimension and Structure Sizesupporting

confidence: 80%

Section: Feature Identificationmentioning

confidence: 99%

Section: Fractal Dimension Definitionmentioning

confidence: 99%

See 2 more Smart Citations

On the reliability of the fractal dimension measure of solar magnetic features and on its variation with solar activity

Criscuoli

Rast

Ermolli³

et al. 2006

A&A

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“…In this context, investigation of fractal and multifractal properties of photospheric magnetic features provide important information about the physical properties of convective plasma (Balke et al 1993;Tao et al 1995;Nesme-Ribes et al 1996), of the magnetic field (Stenflo & Holzreuter 2002;Pietarila Graham et al 2009) and about the physical processes that govern their interaction (Lawrence et al 1995(Lawrence et al , 1999Bushby & Houghton 2005;Crouch et al 2007). Studies carried out on short temporal ranges (hours-days), have showed that fractal and multifractal properties vary with the evolution of the photospheric magnetic field of the regions analyzed (e.g.…”

Section: Introductionmentioning

confidence: 99%

Magnetic evolution of superactive regions

Criscuoli¹,

Romano²,

Zuccarello³

et al. 2009

A&A

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Context. It is widely accepted that solar flares are manifestations of magnetic reconnection events taking place in the solar atmosphere. Several aspects of these events remain unclear, although many efforts have been devoted to the investigation of magnetic field configurations at flare occurrence sites. Aims. In this work, we have studied the temporal evolution of some properties of a sample of superactive regions with the aim to single out the most significant for flare activity forecasting. Methods. We have investigated properties of 14 superactive regions, observed between January 1st 2000 and December 31st 2006 with MDI/SOHO instrument and characterized by a particularly intense flare activity during their passage on the solar disk. We have analyzed the temporal evolution of fractal and multifractal properties of photospheric magnetic fields, namely the generalized fractal dimension and the contribution and dimensional diversities, which describe geometrical properties of the magnetic field, as well as the potential unstable volumes of magnetic discontinuities above the studied ARs, which may provide information about the magnetic field configuration in upper layers of the atmosphere. Correlations of these quantities with the flare index, which provides information about the flare activity of a region, have also been estimated. Results. We found that in 50% of our sample the generalized fractal dimension is correlated with the flare index computed over windows of 50 h, while the contribution diversity and the dimensional diversity are anticorrelated with the same index. A clear increase of the potential unstable volume of magnetic discontinuities in the corona is observed before the phases characterized by more frequent and intense flares. We also found that the free energy distribution functions of unstable volumes of the analyzed superactive regions can be fitted with straight lines whose slope is larger than the values found in previous works for less active magnetic regions. Conclusions. The generalized fractal dimension and the potential unstable volume of magnetic discontinuities are the most suitable for statistical investigations of relations with flare activity over longer (50 h) and shorter (few hours) time intervals, respectively.

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Aeronomy of the Middle Atmosphere

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The Stellar Dynamo

Cited by 25 publications

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On the reliability of the fractal dimension measure of solar magnetic features and on its variation with solar activity

On the reliability of the fractal dimension measure of solar magnetic features and on its variation with solar activity

Magnetic evolution of superactive regions

Radiation

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