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BIPOLAR MAGNETIC SPOTS FROM DYNAMOS IN STRATIFIED SPHERICAL SHELL TURBULENCE
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0002-7304-021X
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0001-6162-7112
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2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 2, 166Article in journal (Refereed) Published
Abstract [en]

Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75 degrees latitude and an azimuthal extent of 180 degrees. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker's Omega-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10 degrees. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.

Place, publisher, year, edition, pages
2015. Vol. 805, no 2, 166
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Physical Sciences
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URN: urn:nbn:se:kth:diva-171150DOI: 10.1088/0004-637X/805/2/166ISI: 000356715400081Scopus ID: 2-s2.0-84930635401OAI: oai:DiVA.org:kth-171150DiVA: diva2:842405
Note

QC 20150720

Available from: 2015-07-20 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved

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Brandenburg, AxelMitra, Dhrubaditya

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Jabbari, SarahBrandenburg, AxelKleeorin, NathanMitra, DhrubadityaRogachevskii, Igor
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