Magnetic helicity

In plasma physics, magnetic helicity is a measure of the linkage, twist, and writhe of a magnetic field. In ideal magnetohydrodynamics, magnetic helicity is conserved. When a magnetic field contains magnetic helicity, it tends to form large-scale structures from small-scale ones. This process can be referred to as an inverse transfer in Fourier space. This property of increasing the scale of structures makes magnetic helicity special, as three-dimensional turbulent flows in ordinary fluid mechanics tend to "destroy" structure, in the sense that large-scale vortices break up into smaller ones, until dissipating through viscous effects into heat. Through a parallel but inverted process, the opposite happens for magnetic vortices, where small helical structures with non-zero magnetic helicity combine and form large-scale magnetic fields. This is visible in the dynamics of the heliospheric current sheet, a large magnetic structure in the Solar System.

Magnetic helicity is a significant concept in the analysis of astrophysical systems, where the resistivity may be very low so that magnetic helicity is conserved to a very good approximation. For example, magnetic helicity dynamics are important in analyzing solar flares and coronal mass ejections. Magnetic helicity is present in the solar wind. Its conservation is significant in dynamo processes. It also plays a role in fusion research, for example in reversed field pinch experiments.