Action Principles for Relativistic Extended Magnetohydrodynamics: A Unified Theory of Magnetofluid Models

TL;DR

This paper develops unified action principles for relativistic extended magnetohydrodynamics, introduces a Hamiltonian formulation for relativistic Hall MHD with electron thermal inertia, and derives a generalized Poisson bracket for nonrelativistic extended MHD.

Contribution

It formulates two types of Eulerian action principles for relativistic extended MHD, introduces the Hamiltonian formulation for relativistic Hall MHD with thermal inertia, and derives a generalized Poisson bracket for nonrelativistic extended MHD.

Findings

Action principles lead to relativistic Hall and ideal MHD limits.

First Hamiltonian formulation of relativistic Hall MHD with thermal inertia.

Derived a generalized 3+1 Poisson bracket for nonrelativistic extended MHD.

Abstract

Two types of Eulerian action principles for relativistic extended magnetohydrodynamics (MHD) are formulated. With the first, the action is extremized under the constraints of density, entropy, and Lagrangian label conservation, which leads to a Clebsch representation for a generalized momentum and a generalized vector potential. The second action arises upon transformation to physical field variables, giving rise to a covariant bracket action principle, i.e., a variational principle in which constrained variations are generated by a degenerate Poisson bracket. Upon taking appropriate limits, the action principles lead to relativistic Hall MHD and well-known relativistic ideal MHD. For the first time, the Hamiltonian formulation of relativistic Hall MHD with electron thermal inertia (akin to [Comisso \textit{et al.}, Phys. Rev. Lett. {\bf 113}, 045001 (2014)] for the electron--positron plasma) is introduced. This thermal inertia effect allows for violation of the frozen-in magnetic flux condition in marked contrast to nonrelativistic Hall MHD that does satisfy the frozen-in condition. We also find violation of the frozen-in condition is accompanied by freezing-in of an alternative flux determined by a generalized vector potential. Finally, we derive a more general 3+1 Poisson bracket for nonrelativistic extended MHD, one that does not assume smallness of the electron ion mass ratio.