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In plasma physics, magnetic helicity is a measure of the linkage, twist, and writhe of a magnetic field.[1][2]
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 good approximation. In practice, magnetic helicity dynamics are important in analyzing solar flares and coronal mass ejections.[3] Magnetic helicity is present in 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.
^Cantarella, Jason; Deturck, Dennis; Gluck, Herman; Teytel, Mikhail (2013-03-19). "Influence of Geometry and Topology on Helicity". Magnetic Helicity in Space and Laboratory Plasmas. Washington, D. C.: American Geophysical Union. pp. 17–24. doi:10.1029/gm111p0017. ISBN 978-1-118-66447-6. Retrieved 2021-01-18.
^Moffatt, H. K. (1969-01-16). "The degree of knottedness of tangled vortex lines". Journal of Fluid Mechanics. 35 (1): 117–129. Bibcode:1969JFM....35..117M. doi:10.1017/s0022112069000991. ISSN 0022-1120. S2CID 121478573.
^Low, B. C. (1996). "Magnetohydrodynamic Processes in the Solar Corona: Flares, Coronal Mass Ejections and Magnetic Helicity". Solar and Astrophysical Magnetohydrodynamic Flows. Dordrecht: Springer Netherlands. pp. 133–149. doi:10.1007/978-94-009-0265-7_7. ISBN 978-94-010-6603-7. Retrieved 2020-10-08.
^Bieber, J. W.; Evenson, P. A.; Matthaeus, W. H. (April 1987). "Magnetic helicity of the Parker field". The Astrophysical Journal. 315: 700. Bibcode:1987ApJ...315..700B. doi:10.1086/165171. ISSN 0004-637X.
^Blackman, E.G. (2015). "Magnetic Helicity and Large Scale Magnetic Fields: A Primer". Space Science Reviews. 188 (1–4): 59–91. arXiv:1402.0933. Bibcode:2015SSRv..188...59B. doi:10.1007/s11214-014-0038-6. S2CID 17015601.
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Brandenburg, A. (2009). "Hydromagnetic Dynamo Theory". Scholarpedia. 2 (3): 2309. Bibcode:2007SchpJ...2.2309B. doi:10.4249/scholarpedia.2309. rev #73469.
^Frisch, U.; Pouquet, A.; LÉOrat, J.; Mazure, A. (1975-04-29). "Possibility of an inverse cascade of magnetic helicity in magnetohydrodynamic turbulence". Journal of Fluid Mechanics. 68 (4): 769–778. Bibcode:1975JFM....68..769F. doi:10.1017/s002211207500122x. ISSN 0022-1120. S2CID 45460069.
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Berger, M.A. (1999). "Introduction to magnetic helicity". Plasma Physics and Controlled Fusion. 41 (12B): B167–B175. Bibcode:1999PPCF...41B.167B. doi:10.1088/0741-3335/41/12B/312. S2CID 250734282.
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