Journal Article

Non-ideal evolution of non-axisymmetric, force-free magnetic fields in a magnetar

A. Mastrano and A. Melatos

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 387, issue 4, pages 1735-1744
Published in print July 2008 | ISSN: 0035-8711
Published online July 2008 | e-ISSN: 1365-2966 | DOI:
Non-ideal evolution of non-axisymmetric, force-free magnetic fields in a magnetar

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Recent numerical magnetohydrodynamic calculations by Braithwaite and collaborators support the ‘fossil field’ hypothesis regarding the origin of magnetic fields in compact stars and suggest that the resistive evolution of the fossil field can explain the reorganization and decay of magnetar magnetic fields. Here, these findings are modelled analytically by allowing the stellar magnetic field to relax through a quasi-static sequence of non-axisymmetric, force-free states, by analogy with spheromak relaxation experiments, starting from a random field. Under the hypothesis that the force-free modes approach energy equipartition in the absence of resistivity, the output of the numerical calculations is semiquantitatively recovered: the field settles down to a linked poloidal–toroidal configuration, which inflates and becomes more toroidal as time passes. A qualitatively similar (but not identical) end state is reached if the magnetic field evolves by exchanging helicity between small and large scales according to an α-dynamo-like, mean-field mechanism, arising from the fluctuating electromotive force produced by the initial random field. The impossibility of matching a force-free internal field to a potential exterior field is discussed in the magnetar context.

Keywords: MHD; plasmas; stars: magnetic fields; stars: neutron

Journal Article.  7406 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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