Anthony Yeates | The Royal Astronomical Society

Gather.town id

CDH07

Poster Title

Revisiting Taylor relaxation

Institution

Durham University

Abstract (short summary)

Turbulent magnetic relaxation is an important candidate mechanism for coronal heating and some types of solar flare. By developing turbulence that reconnects the magnetic field throughout a large volume, magnetic fields can spontaneously self-organize into simpler lower-energy configurations. We are using resistive MHD simulations to probe this relaxation process, in particular to test whether a linear force-free equilibrium is reached. Such an end state would be predicted if one assumes the classic Taylor hypothesis: that the only constraints on the relaxation come from conservation of total magnetic flux and helicity. In fact, a linear force-free state is not reached in our simulations, despite the conservation of these total quantities. Instead, the end state is better characterised as a state of (locally) uniform field-line helicity.

Plain text (extended) Summary

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.

URL

https://www.maths.dur.ac.uk/~bmjg46/