Change search
ReferencesLink to record
Permanent link

Direct link
The α effect with imposed and dynamo-generated magnetic fields
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
2009 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 398, no 4, 1891-1899 p.Article in journal (Refereed) Published
Abstract [en]

Estimates for the non-linear α effect in helical turbulence with an applied magnetic field are presented using two different approaches: the imposed-field method where the electromotive force owing to the applied field is used, and the test-field method where separate evolution equations are solved for a set of different test fields. Both approaches agree for stronger fields, but there are apparent discrepancies for weaker fields that can be explained by the influence of dynamo-generated magnetic fields on the scale of the domain that are referred to as meso-scale magnetic fields. Examples are discussed where these meso-scale fields can lead to both drastically overestimated and underestimated values of α compared with the kinematic case. It is demonstrated that the kinematic value can be recovered by resetting the fluctuating magnetic field to zero in regular time intervals. It is concluded that this is the preferred technique both for the imposed-field and the test-field methods.

Place, publisher, year, edition, pages
2009. Vol. 398, no 4, 1891-1899 p.
Keyword [en]
hydrodynamics, magnetic fields, MHD, turbulence
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-32538DOI: 10.1111/j.1365-2966.2009.15108.xISI: 000269906400023OAI: diva2:280882
Available from: 2009-12-12 Created: 2009-12-12 Last updated: 2016-07-01Bibliographically 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

Open Access in DiVA

No full text

Other links

Publisher's full text

Search in DiVA

By author/editor
Del Sordo, FabioBrandenburg, Axel
By organisation
Department of AstronomyNordic Institute for Theoretical Physics (Nordita)
In the same journal
Monthly notices of the Royal Astronomical Society
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 62 hits
ReferencesLink to record
Permanent link

Direct link