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Magnetic flux concentrations from dynamo-generated fields
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.ORCID iD: 0000-0002-7304-021X
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Ben-Gurion University of the Negev, Israel .
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 568, A112- p.Article in journal (Refereed) Published
Abstract [en]

Context The mean field theory of magnetized stellar convection gives rise to two distinct instabilities; the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an alpha(2) mean field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods. We use both DNS and mean field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal held, We use the test-field method to compute relevant dynamo coefficients. Results. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MPS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.

Place, publisher, year, edition, pages
2014. Vol. 568, A112- p.
Keyword [en]
sunspots, dynamo, turbulence, magnetohydrodynamics (MHD), hydrodynamics
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-154393DOI: 10.1051/0004-6361/201423499ISI: 000341185900042Scopus ID: 2-s2.0-84906854826OAI: oai:DiVA.org:kth-154393DiVA: diva2:756861
Funder
EU, European Research Council, 227952Swedish Research Council, 621-2011-5076 2012-5797
Note

QC 20141020

Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved

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Brandenburg, Axel

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Jabbari, SarahBrandenburg, AxelRivero Losada, IllaKleeorin, NathanRogachevskii, Igor
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