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Active Region Formation through the Negative Effective Magnetic Pressure Instability
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
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2013 (English)In: Solar Physics, ISSN 0038-0938, E-ISSN 1573-093X, Vol. 287, no 1-2, 293-313 p.Article in journal (Refereed) Published
Abstract [en]

The negative effective magnetic-pressure instability operates on scales encompassing many turbulent eddies, which correspond to convection cells in the Sun. This instability is discussed here in connection with the formation of active regions near the surface layers of the Sun. This instability is related to the negative contribution of turbulence to the mean magnetic pressure that causes the formation of large-scale magnetic structures. For an isothermal layer, direct numerical simulations and mean-field simulations of this phenomenon are shown to agree in many details, for example the onset of the instability occurs at the same depth. This depth increases with increasing field strength, such that the growth rate of this instability is independent of the field strength, provided the magnetic structures are fully contained within the domain. A linear stability analysis is shown to support this finding. The instability also leads to a redistribution of turbulent intensity and gas pressure that could provide direct observational signatures.

Place, publisher, year, edition, pages
2013. Vol. 287, no 1-2, 293-313 p.
Keyword [en]
Magnetohydrodymanics, Sun:dynamo, Sunspots
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-80814DOI: 10.1007/s11207-012-0031-8ISI: 000324115600019OAI: oai:DiVA.org:su-80814DiVA: diva2:557729
Funder
EU, European Research Council, 227952Swedish Research Council, 621-2011-5076
Available from: 2012-09-28 Created: 2012-09-28 Last updated: 2017-12-07Bibliographically approved
In thesis
1. From mean-field hydromagnetics to solar magnetic flux concentrations
Open this publication in new window or tab >>From mean-field hydromagnetics to solar magnetic flux concentrations
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main idea behind the work presented in this thesis is to investigate if it is possible to find a mechanism that leads to surface magnetic field concentrations and could operate under solar conditions without postulating the presence of magnetic flux tubes rising from the bottom of the convection zone, a commonly used yet physically problematic approach.

In this context we study the ‘negative effective magnetic pressure effect’: it was pointed out in earlier work (Kleeorin et al., 1989) that the presence of a weak magnetic field can lead to a reduction of the mean turbulent pressure on large length scales. This reduction is now indeed clearly observed in simulations.

As magnetic fluctuations experience an unstable feedback through this effect, it leads, in a stratified medium, to the formation of magnetic structures, first observed numerically in the fifth paper of this thesis. While our setup is relatively simple, one wonders if this instability, as a mechanism able to concentrate magnetic fields in the near surface layers, may play a role in the formation of sunspots, starting from a weak dynamo-generated field throughout the convection zone rather than from strong flux tubes stored at the bottom.

A generalization of the studied case is ongoing.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2012. 51 p.
Keyword
magneto-hydrodynamics, solar physics, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-80817 (URN)978-91-7447-576-0 (ISBN)
Public defence
2012-10-26, sal FB42 , AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:30 (English)
Opponent
Supervisors
Note

At the time of the the doctoral defence the following paper was unpublished and had a status as follows: Paper nr 7: Submitted

Available from: 2012-10-04 Created: 2012-09-28 Last updated: 2012-10-01Bibliographically approved

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