Abstract
We investigate the stability properties for a family of equations introduced by Moffatt to model magnetic relaxation. These models preserve the topology of magnetic streamlines, contain a cubic nonlinearity, and yet have a favorable \(L^2\) energy structure. We consider the local and global in time well-posedness of these models and establish a difference between the behavior as \(t\rightarrow \infty \) with respect to weak and strong norms.
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Notes
Here, we say that \(B_1\) and \(B_0\) are topologically equivalent, if \(B_1(X(\alpha )) = \nabla _\alpha X(\alpha ) B_0(\alpha )\) for a volume preserving diffeomorphism \(\alpha \mapsto X(\alpha )\). In contrast, to say that \(B_1\) is topologically accessible from \(B_0\) means that (see e.g. in [Mof21, Section 8.2.1]) \(B_1 = \lim _{t\rightarrow \infty } B(\cdot ,t)\), where B is a solution of (1.3a) with initial datum \(B_0\) and some solenoidal vector field u, under the additional property that \(\int _0^\infty \left| \int _{{\mathbb {T}}^d} B \cdot (B\cdot \nabla u) dx \right| dt < \infty \).
Note in contrast that the cross-helicity \(\int _{{\mathbb {T}}^d} u\cdot B dx\) is expected to vanish as \(t\rightarrow \infty \) since \(B(\cdot ,t)\) remains uniformly bounded in \(L^2\), while \(u(\cdot ,t)\rightarrow 0\) in \(L^2\).
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Acknowledgements
R.B. was supported by the NSF Graduate Fellowship Grant 1839302. S.F. was in part supported by NSF grant DMS 1613135. S.F. thanks IAS for its hospitality when she was a Member in 2020-21. V.V. was in part supported by the NSF grant CAREER DMS 1911413. V.V. thanks B. Texier and S. Shkoller for stimulating discussions.
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Beekie, R., Friedlander, S. & Vicol, V. On Moffatt’s Magnetic Relaxation Equations. Commun. Math. Phys. 390, 1311–1339 (2022). https://doi.org/10.1007/s00220-021-04289-3
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DOI: https://doi.org/10.1007/s00220-021-04289-3