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Evolution of Field Line Helicity in Magnetic Relaxation

Yeates, A.R. and Russell, A.J.B. and Hornig, G. (2021) 'Evolution of Field Line Helicity in Magnetic Relaxation.', Physics of plasmas., 28 (8). 082904.


Plasma relaxation in the presence of an initially braided magnetic field can lead to self-organization into relaxed states that retain non-trivial magnetic structure. These relaxed states may be in conflict with the linear force-free fields predicted by the classical Taylor theory, and remain to be fully understood. Here, we study how the individual field line helicities evolve during such a relaxation, and show that they provide new insights into the relaxation process. The line helicities are computed for numerical resistive-magnetohydrodynamic simulations of a relaxing braided magnetic field with line-tied boundary conditions, where the relaxed state is known to be non-Taylor. Firstly, our computations confirm recent analytical predictions that line helicity will be predominantly redistributed within the domain, rather than annihilated. Secondly, we show that self-organization into a relaxed state with two discrete flux tubes may be predicted from the initial line helicity distribution. Thirdly, for this set of line-tied simulations we observe that the sub-structure within each of the final tubes is a state of uniform line helicity. This uniformization of line helicity is consistent with Taylor theory applied to each tube individually. However, it is striking that the line helicity becomes significantly more uniform than the force-free parameter.

Item Type:Article
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Available under License - Creative Commons Attribution 4.0.
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Publisher statement:© 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://
Date accepted:01 August 2021
Date deposited:04 August 2021
Date of first online publication:18 August 2021
Date first made open access:04 August 2021

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