In plasma physics, magnetic helicity is the extent to which a magnetic field "wraps around itself". It is a generalization of the topological concept of linking number to the differential quantities required to describe the magnetic field. As with many quantities in electromagnetism, magnetic helicity (which describes magnetic field lines) is closely related to fluid mechanical helicity (which describes fluid flow lines).
If magnetic field lines follow the strands of a twisted rope, this configuration would have nonzero magnetic helicity; lefthanded ropes would have negative values and righthanded ropes would have positive values.
Formally,
where
Magnetic helicity is a conserved quantity. It is conserved in electromagnetic fields, even when magnetic reconnection dissipates energy. The concept is useful in solar dynamics and in hydromagnetic dynamo theory.
Magnetic helicity is a gaugedependent quantity, because can be redefined by adding a gradient to it (gauge transformation). However, for perfectly conducting boundaries or periodic systems without a net magnetic flux, the magnetic helicity is gauge invariant. A gaugeinvariant relative helicity has been defined for volumes with nonzero magnetic flux on their boundary surfaces (e.g., Berger 1999). If the magnetic field is turbulent and weakly inhomogeneous a magnetic helicity density and its associated flux can be defined in terms of the density of field line linkages.
