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Constraints on dynamo action in plasmas

Published online by Cambridge University Press:  02 November 2016

P. Helander ,
M. Strumik  and
A. A. Schekochihin
P. Helander*
Affiliation:
Max-Planck-Institut für Plasmaphysik, 17491 Greifswald, Germany Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK Merton College, Oxford OX1 4JD, UK
M. Strumik
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK
A. A. Schekochihin
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK Merton College, Oxford OX1 4JD, UK
*
†Email address for correspondence: per.helander@ipp.mpg.de
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Abstract

Upper bounds are derived on the amount of magnetic energy that can be generated by dynamo action in collisional and collisionless plasmas with and without external forcing. A hierarchy of mathematical descriptions is considered for the plasma dynamics: ideal magnetohydrodynamics (MHD), visco-resistive MHD, the double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD and other kinetic models. It is found that dynamo action is greatly constrained in models where the magnetic moment of any particle species is conserved. In the absence of external forcing, the magnetic energy then remains small at all times if it is small in the initial state. In other words, a small ‘seed’ magnetic field cannot be amplified significantly, regardless of the nature of flow, as long as the collision frequency and gyroradius are small enough to be negligible. A similar conclusion also holds if the system is subject to external forcing as long as this forcing conserves the magnetic moment of at least one plasma species and does not greatly increase the total energy of the plasma (i.e. in practice, is subsonic). Dynamo action therefore always requires collisions or some small-scale kinetic mechanism for breaking the adiabatic invariance of the magnetic moment.

Keywords

astrophysical plasmasmagnetised plasmas
Type
Research Article
Information
Journal of Plasma Physics , Volume 82 , Issue 6 , December 2016 , 905820601
Copyright
© Cambridge University Press 2016 

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