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Detection of Negative Effective Magnetic Pressure Instability in Turbulence Simulations
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
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2011 (English)In: Astrophysical Journal, ISSN 0004-637X, EISSN 1538-4357, Vol. 740, no 2, L50- p.Article in journal (Refereed) Published
Abstract [en]

We present the first numerical demonstration of the negative effective magnetic pressure instability in direct numerical simulations of stably stratified, externally forced, isothermal hydromagnetic turbulence in the regime of large plasma beta. By the action of this instability, initially uniform horizontal magnetic field forms flux concentrations whose scale is large compared to the turbulent scale. We further show that the magnetic energy of these large-scale structures is only weakly dependent on the magnetic Reynolds number. Our results support earlier mean-field calculations and analytic work that identified this instability. Applications to the formation of active regions in the Sun are discussed.

Place, publisher, year, edition, pages
2011. Vol. 740, no 2, L50- p.
Keyword [en]
magnetohydrodynamics: MHD, Sun: dynamo, sunspots, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-70035DOI: 10.1088/2041-8205/740/2/L50ISI: 000296752600019OAI: oai:DiVA.org:su-70035DiVA: diva2:478731
Note

authorCount :6;

Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2012-09-30Bibliographically 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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