Magnetic helicity

In plasma physics, magnetic helicity is a measure of the linkage, twist, and writhe of a magnetic field.

Magnetic helicity is used to analyze systems with very low resistivity, including many astrophysical environments. When resistivity is low, magnetic helicity is approximately conserved over long timescales. Magnetic helicity dynamics are important in studies of solar flares and coronal mass ejections. It is relevant in the dynamics of the solar wind. Its approximate conservation is significant in dynamo processes. It also plays a role in fusion research, including reversed field pinch experiments.

When a magnetic field contains magnetic helicity, it can drive the formation of large-scale structures from small-scale ones. This process is referred to as inverse transfer in Fourier space. In three dimensions, magnetic helicity supports growth toward larger scales. In contrast, many three-dimensional flows in ordinary fluid mechanics are turbulent and exhibit a direct cascade in which large-scale vortices break up into smaller ones that dissipate through viscous effects. By a parallel but inverted process, small helical magnetic structures with nonzero magnetic helicity combine to form large-scale magnetic fields. This behavior is observed in the dynamics of the heliospheric current sheet, a large magnetic structure in the Solar System.