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Model of driven and decaying magnetic turbulence in a cylinder
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.
2011 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 84, 56407- p.Article in journal (Refereed) Published
Abstract [en]

Using mean-field theory, we compute the evolution of the magnetic field in a cylinder with outer perfectly conducting boundaries and imposed axial magnetic and electric fields. The thus injected magnetic helicity in the system can be redistributed by magnetic helicity fluxes down the gradient of the local current helicity of the small-scale magnetic field. A weak reversal of the axial magnetic field is found to be a consequence of the magnetic helicity flux in the system. Such fluxes are known to alleviate so-called catastrophic quenching of the α effect in astrophysical applications. A stronger field reversal can be obtained if there is also a significant kinetic α effect. Application to the reversed field pinch in plasma confinement devices is discussed.

Place, publisher, year, edition, pages
2011. Vol. 84, 56407- p.
Keyword [en]
Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps, Current drive, helicity injection, Magnetohydrodynamic and fluid equation, Sun spots, solar cycles
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-70043DOI: 10.1103/PhysRevE.84.056407ISI: 000297467500005OAI: oai:DiVA.org:su-70043DiVA: diva2:478717
Note

authorCount: 3;

Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2017-12-08Bibliographically 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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Kemel, KoenBrandenburg, Axel
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