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Advances in mean-field dynamo theory and applications to astrophysical turbulence

Part of: Focus on Plasma Astrophysics 50 years of Mean Field Electrodynamics

Published online by Cambridge University Press:  07 August 2018

Axel Brandenburg Open the ORCID record for Axel Brandenburg [Opens in a new window]
Axel Brandenburg*
Affiliation:
Laboratory for Atmospheric and Space Physics, JILA, and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80303, USA Nordita, KTH Royal Institute of Technology and Stockholm University, and Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden
*
†Email address for correspondence: brandenb@nordita.org
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Abstract

Recent advances in mean-field theory are reviewed and applications to the Sun, late-type stars, accretion disks, galaxies and the early Universe are discussed. We focus particularly on aspects of spatio-temporal non-locality, which provided some of the main new qualitative and quantitative insights that emerged from applying the test-field method to magnetic fields of different length and time scales. We also review the status of nonlinear quenching and the relation to magnetic helicity, which is an important observational diagnostic of modern solar dynamo theory. Both solar and some stellar dynamos seem to operate in an intermediate regime that has not yet been possible to model successfully. This regime is bracketed by antisolar-like differential rotation on one end and stellar activity cycles belonging to the superactive stars on the other. The difficulty in modelling this regime may be related to shortcomings in simulating solar/stellar convection. On galactic and extragalactic length scales, the observational constraints on dynamo theory are still less stringent and more uncertain, but recent advances both in theory and observations suggest that more conclusive comparisons may soon be possible also here. The possibility of inversely cascading magnetic helicity in the early Universe is particularly exciting in explaining the recently observed lower limits of magnetic fields on cosmological length scales. Such magnetic fields may be helical with the same sign of magnetic helicity throughout the entire Universe. This would be a manifestation of parity breaking.

Keywords

astrophysical plasmas
Type
Research Article
Information
Journal of Plasma Physics , Volume 84 , Issue 4 , August 2018 , 735840404
Copyright
© Cambridge University Press 2018 

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