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Surface flux concentrations in a spherical alpha 2 dynamo
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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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, A106- p.Article in journal (Refereed) Published
Abstract [en]

Context. In the presence of strong density stratification, turbulence can lead to the large-scale instability of a horizontal magnetic field if its strength is in a suitable range (around a few percent of the turbulent equipartition value). This instability is related to a suppression of the turbulent pressure so that the turbulent contribution to the mean magnetic pressure becomes negative. This results in the excitation of a negative effective magnetic pressure instability (NEMPI). This instability has so far only been studied for an imposed magnetic field. Aims. We want to know how NEMPI works when the mean magnetic field is generated self-consistently by an alpha(2) dynamo, whether it is affected by global spherical geometry, and whether it can influence the properties of the dynamo itself. Methods. We adopt the mean-field approach, which has previously been shown to provide a realistic description of NEMPI in direct numerical simulations. We assume axisymmetry and solve the mean-field equations with the Pencil Code for an adiabatic stratification at a total density contrast in the radial direction of approximate to 4 orders of magnitude. Results. NEMPI is found to work when the dynamo-generated field is about 4% of the equipartition value, which is achieved through strong alpha quenching. This instability is excited in the top 5% of the outer radius, provided the density contrast across this top layer is at least 10. NEMPI is found to occur at lower latitudes when the mean magnetic field is stronger. For weaker fields, NEMPI can make the dynamo oscillatory with poleward migration. Conclusions. NEMPI is a viable mechanism for producing magnetic flux concentrations in a strongly stratified spherical shell in which a magnetic field is generated by a strongly quenched alpha effect dynamo.

Place, publisher, year, edition, pages
2013. Vol. 556, A106- p.
Keyword [en]
sunspots, Sun: dynamo, turbulence, magnetohydrodynamics (MHD), hydrodynamics
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-129629DOI: 10.1051/0004-6361/201321353ISI: 000323893500106ScopusID: 2-s2.0-84881240285OAI: diva2:653201
EU, European Research Council, 227952 227915

QC 2013103

Available from: 2013-10-03 Created: 2013-10-03 Last updated: 2013-10-03Bibliographically approved

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Jabbari, SarahBrandenburg, AxelKleeorin, NathanMitra, DhrubadityaRogachevskii, Igor
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