Magnetic reconnection and dynamos in the presence of plasma turbulence

TL;DR

This paper discusses how Maxwell's equations explain the widespread occurrence of fast magnetic reconnection in turbulent plasmas and highlights constraints on mean-field theories of magnetic field evolution, emphasizing the importance of three-dimensional effects.

Contribution

It clarifies the fundamental constraints from Maxwell's equations on magnetic reconnection and mean-field magnetohydrodynamics in turbulent plasmas, challenging two-dimensional assumptions.

Findings

Maxwell's equations explain the ubiquity of fast reconnection.

Constraints on mean-field theories are derived from fundamental electromagnetic principles.

Three-dimensional effects are crucial, unlike in simplified two-dimensional models.

Abstract

Evolving magnetic fields are frequently embedded in plasmas that are turbulent. When the primary interest is in effects that are on a large scale compared to that of the turbulence, it is desirable to average over the turbulence to obtain equations for mean-field magnetohydrodynamics. An obvious constraint on the validity of the averaging is that large-scale quantities that evolve slowly using the exact evolution equations must remain slowly evolving in the mean-field theory. Magnetic helicity is the primary example of such a quantity and maintaining its slow evolution has been controversial in mean-field magnetohydrodynamics. A full theory of magnetic reconnection in turbulent plasmas is not the intent of this paper. What is the intent is to show how exact results from Maxwell's equations explain why fast reconnection is so ubiquitous and what constraints these results place on the theory of magnetic field evolution, including dynamos, whether the plasma is turbulent or not. These constraints are commonly broken in the reconnection literature, which has been heavily influenced by two-dimensional theory that is not applicable to three-dimensional problems.