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Magnetic nulls in interacting dipolar fields

Published online by Cambridge University Press:  03 May 2021

Todd Elder Open the ORCID record for Todd Elder [Opens in a new window]  and
Allen H. Boozer
Todd Elder*
Affiliation:
Department of Applied Physics and Mathematics, Columbia University, New York, NY10027, USA
Allen H. Boozer*
Affiliation:
Department of Applied Physics and Mathematics, Columbia University, New York, NY10027, USA
*
†Email addresses for correspondence: tme2123@columbia.edu, ahb17@columbia.edu
†Email addresses for correspondence: tme2123@columbia.edu, ahb17@columbia.edu
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Abstract

The prominence of nulls in reconnection theory is due to the expected singular current density and the indeterminacy of field lines at a magnetic null. Electron inertia changes the implications of both features. Magnetic field lines are distinguishable only when their distance of closest approach exceeds a distance $\varDelta _d$. Electron inertia ensures $\varDelta _d\gtrsim c/\omega _{pe}$. The lines that lie within a magnetic flux tube of radius $\varDelta _d$ at the place where the field strength $B$ is strongest are fundamentally indistinguishable. If the tube, somewhere along its length, encloses a point where $B=0$ vanishes, then distinguishable lines come no closer to the null than $\approx (a^2c/\omega _{pe})^{1/3}$, where $a$ is a characteristic spatial scale of the magnetic field. The behaviour of the magnetic field lines in the presence of nulls is studied for a dipole embedded in a spatially constant magnetic field. In addition to the implications of distinguishability, a constraint on the current density at a null is obtained, and the time required for thin current sheets to arise is derived.

Keywords

astrophysical plasmasplasma properties
Type
Research Article
Information
Journal of Plasma Physics , Volume 87 , Issue 2 , April 2021 , 905870225
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Copyright
Copyright © The Author(s), 2021. Published by Cambridge University Press

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