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Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
2014 (English)In: The Astrophysical Journal Letters, ISSN 2041-8205, Vol. 784, no 2, L45- p.Article in journal (Refereed) Published
Abstract [en]

We compute for the first time the magnetic helicity and energy spectra of the solar active regionNOAA 11158 during 2011 February 11-15 at 20 degrees southern heliographic latitude using observational photospheric vector magnetograms. We adopt the isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field. The sign of the magnetic helicity turns out to be predominantly positive at all wavenumbers. This sign is consistent with what is theoretically expected for the southern hemisphere. The magnetic helicity normalized to its theoretical maximum value, here referred to as relative helicity, is around 4% and strongest at intermediate wavenumbers of k approximate to 0.4Mm(-1), corresponding to a scale of 2 pi/k approximate to 16 Mm. The same sign and a similar value are also found for the relative current helicity evaluated in real space based on the vertical components of magnetic field and current density. The modulus of the magnetic helicity spectrum shows a k(-11/3) power law at large wavenumbers, which implies a k(-5/3) spectrum for the modulus of the current helicity. A k(-5/3) spectrum is also obtained for the magnetic energy. The energy spectra evaluated separately from the horizontal and vertical fields agree for wavenumbers below 3 Mm(-1), corresponding to scales above 2 Mm. This gives some justification to our assumption of isotropy and places limits resulting from possible instrumental artifacts at small scales.

Place, publisher, year, edition, pages
2014. Vol. 784, no 2, L45- p.
Keyword [en]
dynamo, Sun: activity, Sun: magnetic fields, sunspots, turbulence
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-103975DOI: 10.1088/2041-8205/784/2/L45ISI: 000333333400028OAI: diva2:721810
EU, European Research Council, 227952Swedish Research Council, 2012-5797 621-2011-5076


Available from: 2014-06-05 Created: 2014-05-27 Last updated: 2014-06-05Bibliographically approved

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Brandenburg, Axel
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Nordic Institute for Theoretical Physics (Nordita)Department of Astronomy
Astronomy, Astrophysics and Cosmology

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