Effective Field Theories in Magnetohydrodynamics

Abstract

We briefly review the recent developments in magnetohydrodynamics, which in particular deal with the evolution of magnetic fields in turbulent plasmas. We especially emphasize (i) the necessity of renormalizing equations of motion in turbulence where velocity and magnetic fields become Hölder singular; (ii) the breakdown of Laplacian determinism (spontaneous stochasticity) for turbulent magnetic fields; and (iii) the possibility of eliminating the notion of magnetic field lines, using instead magnetic path lines as trajectories of Alfvenic wave-packets. These methodologies are then exemplified with their application to the problem of magnetic reconnection -- rapid change in magnetic field pattern that accelerates plasma -- a ubiquitous phenomenon in astrophysics and laboratory plasmas. The necessity of smoothing out rough velocity and magnetic fields on a finite scale L implies that magnetohydrodynamic equations should be regarded as effective field theories with running parameters depending upon the scale L.