Transition from 2D HD to 2D MHD turbulence

TL;DR

This paper studies the critical transition in 2D magneto-hydrodynamic turbulence, revealing how energy cascades shift from inverse to forward as magnetic forcing varies, due to competing processes.

Contribution

It identifies the mechanisms behind the transition from inverse to forward energy cascade in 2D MHD turbulence, highlighting the interplay of hydrodynamic and magnetic effects.

Findings

Critical transition results from two competing cascades.

At marginality, cascades cancel each other.

Phase space diagram of the transition is proposed.

Abstract

We investigate the critical transition from an inverse cascade of energy to a forward energy cascade in a two-dimensional magneto-hydrodynamic flow as the ratio of magnetic to mechanical forcing amplitude is varied. It is found that the critical transition is the result of two competing processes. The first process is due to hydrodynamic interactions, cascades the energy to the large scales. The second process couples small scale magnetic fields to large scale flows transferring the energy back to the small scales via a non-local mechanism. At marginality the two cascades are both present and cancel each other. The phase space diagram of the transition is sketched.