On the edge of an inverse cascade

TL;DR

This paper investigates the critical behavior of inverse cascades in two-dimensional magneto-hydrodynamics, showing that energy flux can vanish at a critical point, leading to a phase transition-like phenomenon.

Contribution

It introduces the concept of a critical point where inverse cascade stops, demonstrated through a computational model of 2D MHD turbulence, revealing universal features of variable inverse cascades.

Findings

Energy flux towards large scales becomes zero at a critical magnetic forcing.

Energy flux scales as a power law near the critical point.

Divergence of fluctuations at the critical point.

Abstract

We demonstrate that systems with a parameter-controlled inverse cascade can exhibit critical behavior for which at the critical value of the control parameter the inverse cascade stops. In the vicinity of such a critical point standard phenomenological estimates for the energy balance will fail since the energy flux towards large length scales becomes zero. We demonstrate these concepts using the computationally tractable model of two-dimensional magneto-hydrodynamics in a periodic box. In the absence of any external magnetic forcing the system reduces to hydrodynamic fluid turbulence with an inverse energy cascade. In the presence of strong magnetic forcing the system behaves as 2D magneto-hydrodynamic turbulence with forward energy cascade. As the amplitude of the magnetic forcing is varied a critical value is met for which the energy flux towards the large scales becomes zero. Close to this point the energy flux scales as a power law with the departure from the critical point and the normalized amplitude of the fluctuations diverges. Similar behavior is observed for the flux of the square vector potential for which no inverse flux is observed for weak magnetic forcing, while a finite inverse flux is observed for magnetic forcing above the critical point. We conjecture that this behavior is generic for systems of variable inverse cascade.