Magnetic helicity

Summary

Magnetic helicity is a number that measures how much the field lines of a magnetic field twist around each other and link with each other. It is the integral of the dot product of a vector potential with the field. For thin flux tubes it counts the linking number of the tubes, weighted by their fluxes, so it is a topological quantity. It is nearly conserved in highly conducting plasmas, which makes it central in solar physics and fusion research.