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Solar and Galactic Magnetic Halo Structure: Force-Free Dynamos?

Department of Physics, Engineering Physics & Astronomy, Queen’s University, KIngston, ON K7L 3N6, Canada
Galaxies 2019, 7(2), 53; https://doi.org/10.3390/galaxies7020053
Received: 14 January 2019 / Revised: 5 April 2019 / Accepted: 18 April 2019 / Published: 3 May 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
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Abstract

Magnetic fields may relax dissipatively to the minimum energy force-free condition whenever they are not constantly created or distorted. We review the axially symmetric solutions for force-free magnetic fields, especially for the non-linear field. A new formulation for the scale invariant state is given. Illustrative examples are shown. Applications to both stellar coronas and galactic halos are possible. Subsequently we study whether such force-free fields may be sustained by classical magnetic dynamo action. Although the answer is ‘not indefinitely’, there may be an evolutionary cycle wherein the magnetic field repeatedly relaxes to the minimum energy condition after a period of substantial growth and distortion. Different force-free dynamos may coexist at different locations. Helicity transfer between scales is studied briefly. A dynamo solution is given for the temporal evolution away from an initial linear force-free magnetic field due to both α 2 and ω terms. This can be used at the sub scale level to create a ‘delayed’ α effect. View Full-Text
Keywords: force-free magnetic fields; scale invariance; magnetic dynamos; Alpha effect force-free magnetic fields; scale invariance; magnetic dynamos; Alpha effect
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Henriksen, R. Solar and Galactic Magnetic Halo Structure: Force-Free Dynamos? Galaxies 2019, 7, 53.

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