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Title:
The α effect with imposed and dynamo-generated magnetic fields
Authors:
Hubbard, A.; Del Sordo, F.; Käpylä, P. J.; Brandenburg, A.
Affiliation:
AA(NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden), AB(NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden; Department of Astronomy, Alba Nova University Center, Stockholm University, SE-10691 Stockholm, Sweden), AC(Observatory Tähtitorninmäki, PO Box 14, University of Helsinki, FI-00014 Helsinki, Finland), AD(NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden; Department of Astronomy, Alba Nova University Center, Stockholm University, SE-10691 Stockholm, Sweden)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 398, Issue 4, pp. 1891-1899. (MNRAS Homepage)
Publication Date:
10/2009
Origin:
MNRAS
Astronomy Keywords:
hydrodynamics , magnetic fields , MHD , turbulence
DOI:
10.1111/j.1365-2966.2009.15108.x
Bibliographic Code:
2009MNRAS.398.1891H

Abstract

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.
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