# Magnetic helicity

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; left-handed ropes would have negative values and right-handed ropes would have positive values.[clarification needed]

Formally,

$H = \int {\mathbf A}\cdot{\mathbf B}\,d^3{\mathbf r}$

where

${\mathbf B}$ is the magnetic field strength
${\mathbf B}=\nabla\times{\mathbf A}$;
$\mathbf A$ is the vector potential of ${\mathbf B}$

Magnetic helicity has units of Wb2 (webers squared). It is a conserved quantity in electromagnetic fields, even when magnetic reconnection dissipates energy. The concept is useful in solar dynamics and in hydromagnetic dynamo theory.[1] Magnetic helicity is a gauge-dependent quantity, because $\mathbf A$ 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 gauge-invariant relative helicity has been defined for volumes with non-zero magnetic flux on their boundary surfaces[2] 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.[3]

## References

1. ^ Brandenburg, A. (2009). "Hydromagnetic Dynamo Theory". Scholarpedia 2 (3): 2309. Bibcode:2007SchpJ...2.2309B. doi:10.4249/scholarpedia.2309. rev #73469.
2. ^ Berger, M.A. (1999). "Introduction to magnetic helicity". Plasma Physics and Controlled Fusion 41 (12B): 167. Bibcode:1999PPCF...41..167B. doi:10.1088/0741-3335/41/12B/312.
3. ^ Subramanian, K.; Brandenburg, A. (2006). "Magnetic helicity density and its flux in weakly inhomogeneous turbulence". The Astrophysical Journal Letters 648: L71–L74. arXiv:astro-ph/0509392. Bibcode:2006ApJ...648L..71S. doi:10.1086/507828.