Magnetic helicity
Measure of magnetic field topology / From Wikipedia, the free encyclopedia
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In plasma physics, magnetic helicity is a measure of the linkage, twist, and writhe of a magnetic field.[1][2]
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Magnetic helicity is a useful concept in the analysis of systems with extremely low resistivity, such as astrophysical systems. When resistivity is low, magnetic helicity is conserved over longer timescales, to a good approximation. Magnetic helicity dynamics are particularly important in analyzing solar flares and coronal mass ejections.[3] Magnetic helicity is relevant in the dynamics of the solar wind.[4] Its conservation is significant in dynamo processes, and it also plays a role in fusion research, such as reversed field pinch experiments.[5][6][7][8][9]
When a magnetic field contains magnetic helicity, it tends to form large-scale structures from small-scale ones.[10] 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 in three dimensions, as other three-dimensional flows in ordinary fluid mechanics are the opposite, being turbulent and having the tendency 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,[11] a large magnetic structure in the Solar System.