Conservation properties and potential systems of vorticity-type equations

Abstract

Partial differential equations of the form \documentclass[12pt]{minimal}\begin{document}$\mathop {\hbox{\rm div}}{{\bm N}} =0$\end{document}divN=0, \documentclass[12pt]{minimal}\begin{document}${{\bm N}}_t + \mathop {\hbox{\rm curl}}{{\bm M}}=0$\end{document}Nt+curlM=0 involving two vector functions in \documentclass[12pt]{minimal}\begin{document}$\mathbb {R}^3$\end{document}R3 depending on t, x, y, z appear in different physical contexts, including the vorticity formulation of fluid dynamics, magnetohydrodynamics (MHD) equations, and Maxwell's equations. It is shown that these equations possess an infinite family of local divergence-type conservation laws involving arbitrary functions of space and time. Moreover, it is demonstrated that the equations of interest have a rather special structure of a lower-degree (degree two) conservation law in \documentclass[12pt]{minimal}\begin{document}$\mathbb {R}^4(t,x,y,z)$\end{document}R4(t,x,y,z). The corresponding potential system has a clear physical meaning. For the Maxwell's equations, it gives rise to the scalar electric and the vector magnetic potentials; for the vorticity equations of fluid dynamics, the potentialization inverts the curl operator to yield the fluid dynamics equations in primitive variables; for MHD equations, the potential equations yield a generalization of the Galas-Bogoyavlenskij potential that describes magnetic surfaces of ideal MHD equilibria. The lower-degree conservation law is further shown to yield curl-type conservation laws and determined potential equations in certain lower-dimensional settings. Examples of new nonlocal conservation laws, including an infinite family of nonlocal material conservation laws of ideal time-dependent MHD equations in 2+1 dimensions, are presented.