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Breakdown of chiral symmetry during saturation of the Tayler instability
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
2012 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, Vol. 86, no 1, 016313- p.Article in journal (Refereed) Published
Abstract [en]

We study spontaneous breakdown of chiral symmetry during the nonlinear evolution of the Tayler instability. We start with an initial steady state of zero helicity. Within linearized perturbation calculations, helical perturbations of this initial state have the same growth rate for either sign of helicity. Direct numerical simulations (DNS) of the fully nonlinear equations, however, show that an infinitesimal excess of one sign of helicity in the initial perturbation gives rise to a saturated helical state. We further show that this symmetry breaking can be described by weakly nonlinear finite-amplitude equations with undetermined coefficients which can be deduced solely from symmetry consideration. By fitting solutions of the amplitude equations to data from DNS, we further determine the coefficients of the amplitude equations.

Place, publisher, year, edition, pages
2012. Vol. 86, no 1, 016313- p.
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-80437DOI: 10.1103/PhysRevE.86.016313ISI: 000306415900005OAI: diva2:555364


Available from: 2012-09-19 Created: 2012-09-19 Last updated: 2012-10-15Bibliographically approved
In thesis
1. From irrotational flows to turbulent dynamos
Open this publication in new window or tab >>From irrotational flows to turbulent dynamos
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many of the celestial bodies we know are found to be magnetized:the Earth, many of the planets so far discovered, the Sun and other stars,the interstellar space, the Milky Way and other galaxies.The reason for that is still to be fully understood, and this work is meant to be a step in that direction.

The dynamics of the interstellar medium is dominated by events likesupernovae explosions that can be modelled as irrotational flows.The first part of this thesis is dedicated to the analysis of some characteristics of these flows, in particular how they influencethe typical turbulent magnetic diffusivity of a medium, and it is shownthat the diffusivity is generally enhanced, except for some specific casessuch as steady potential flows, where it can be lowered.Moreover, it is examined how such flows can develop vorticity when they occur in environments affected by rotation or shear,or that are not barotropic.

Secondly, we examine helical flows, that are of basic importance for the phenomenon of the amplification of magnetic fields, namely the dynamo.Magnetic helicity can arise from the occurrence of an instability: here we focus on theinstability of purely toroidal magnetic fields, also known as Tayler instability.It is possible to give a topological interpretation of magnetic helicity.Using this point of view, and being aware that magnetic helicity is a conserved quantity in non-resistive flows,it is illustrated how helical systems preserve magnetic structureslonger than non-helical ones.

The final part of the thesis deals directly with dynamos.It is shown how to evaluate dynamo transport coefficients with two of the most commonly used techniques, namely theimposed-field and the test-field methods.After that, it is analyzed how dynamos are affected by advectionof magnetic fields and material away from the domain in which theyoperate.It is demonstrated that the presence of an outflow, likestellar or galactic winds in real astrophysical cases,alleviates the so-calledcatastrophic quenching, that is the damping of a dynamoin highly conductive media, thus allowing the dynamo process to work better.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2012. 76 p.
astrophysics, magnetic fields, insterstellar medium, MHD, dynamo, turbulence, instability
National Category
Astronomy, Astrophysics and Cosmology
Research subject
urn:nbn:se:su:diva-80958 (URN)978-91-7447-573-9 (ISBN)
Public defence
2012-11-14, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:15 (English)

At the time of the doctoral defence the following paper was unpublished and had a status as follows: Paper nr 5: Submitted

Available from: 2012-10-23 Created: 2012-10-03 Last updated: 2012-10-17Bibliographically approved

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Brandenburg, AxelDel Sordo, Fabio
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Nordic Institute for Theoretical Physics (Nordita)Department of Astronomy
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Physical Review E. Statistical, Nonlinear, and Soft Matter Physics
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