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The α effect with imposed and dynamo-generated magnetic fields
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
2009 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 398, nr 4, s. 1891-1899Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2009. Vol. 398, nr 4, s. 1891-1899
Nyckelord [en]
hydrodynamics, magnetic fields, MHD, turbulence
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
URN: urn:nbn:se:su:diva-32538DOI: 10.1111/j.1365-2966.2009.15108.xISI: 000269906400023OAI: oai:DiVA.org:su-32538DiVA, id: diva2:280882
Tillgänglig från: 2009-12-12 Skapad: 2009-12-12 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
Ingår i avhandling
1. From irrotational flows to turbulent dynamos
Öppna denna publikation i ny flik eller fönster >>From irrotational flows to turbulent dynamos
2012 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Astronomy, Stockholm University, 2012. s. 76
Nyckelord
astrophysics, magnetic fields, insterstellar medium, MHD, dynamo, turbulence, instability
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-80958 (URN)978-91-7447-573-9 (ISBN)
Disputation
2012-11-14, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:15 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defence the following paper was unpublished and had a status as follows: Paper nr 5: Submitted

Tillgänglig från: 2012-10-23 Skapad: 2012-10-03 Senast uppdaterad: 2012-10-17Bibliografiskt granskad

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Av författaren/redaktören
Del Sordo, FabioBrandenburg, Axel
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Institutionen för astronomiNordiska institutet för teoretisk fysik (Nordita)
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Monthly notices of the Royal Astronomical Society
Astronomi, astrofysik och kosmologi

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