The third order helicity of magnetic fields via link maps

Abstract

We introduce an alternative approach to the third order helicity of a volume preserving vector field $B$, which leads us to a lower bound for the $L^2$-energy of $B$. The proposed approach exploits correspondence between the Milnor ${\bar{\mu }}_{123}$-invariant for 3-component links and the homotopy invariants of maps to configuration spaces, and we provide a simple geometric proof of this fact in the case of Borromean links. Based on these connections we develop a formulation for the third order helicity of $B$ on invariant \emph{unlinked} domains of $B$, and provide Arnold's style ergodic interpretation of this invariant as an average asymptotic ${\bar{\mu }}_{123}$-invariant of orbits of $B$.

30 pages, 5 figures

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