Quasi-two-dimensionality of three-dimensional, magnetically dominated, decaying turbulence

TL;DR

This paper investigates the decay of magnetohydrodynamic turbulence, revealing the role of magnetic reconnection in energy transfer, the quasi-two-dimensional nature of the system, and the applicability of certain invariants in different helicity regimes.

Contribution

It demonstrates the importance of magnetic reconnection in fully helical turbulence and introduces a hierarchical merger model explaining observed power laws.

Findings

Magnetic reconnection drives inverse energy transfer in helical turbulence.

3D turbulence exhibits quasi-two-dimensional characteristics.

A hierarchical merger model reproduces key power law scalings.

Abstract

Decaying magnetohydrodynamic (MHD) turbulence is important in various astrophysical contexts, including early universe magnetic fields, star formation, turbulence in galaxy clusters, magnetospheres and solar corona. Previously known in the nonhelical case of magnetically dominated decaying turbulence, we show that magnetic reconnection is important also in the fully helical case and is likely the agent responsible for the inverse transfer of energy. Again, in the fully helical case, we find that there is a similarity in power law decay exponents in both 2.5D and 3D simulations. To understand this intriguing similarity, we investigate the possible quasi-two-dimensionalization of the 3D system. We perform Minkowski functional analysis and find that the characteristic length scales of a typical magnetic structure in the system are widely different, suggesting the existence of local anisotropies. Finally, we provide a quasi-two-dimensional hierarchical merger model which recovers the relevant power law scalings. In the nonhelical case, we show that a helicity-based invariant cannot constrain the system, and the best candidate is still anastrophy or vector potential squared, which is consistent with the quasi-two-dimensionalization of the system.