Inverse energy transfer in decaying, three dimensional, nonhelical magnetic turbulence due to magnetic reconnection

TL;DR

This study demonstrates that magnetic reconnection drives inverse energy transfer in 3D decaying nonhelical magnetic turbulence, with similar scaling behaviors to 2D cases, and explores the effects of subdominant magnetic fields and dynamo action.

Contribution

It identifies magnetic reconnection as the physical mechanism behind inverse energy transfer in 3D nonhelical turbulence and compares 2D and 3D behaviors, including subdominant magnetic field scenarios.

Findings

Inverse transfer driven by reconnection observed in 3D.

Magnetic energy scales as t^{-1} with a k^{-2} spectrum.

Subdominant magnetic fields show inverse transfer and dynamo effects.

Abstract

It has been recently shown numerically that there exists an inverse transfer of magnetic energy in decaying, nonhelical, magnetically dominated, magnetohydrodynamic turbulence in 3-dimensions (3D). We suggest that magnetic reconnection is the underlying physical mechanism responsible for this inverse transfer. In the two-dimensional (2D) case, the inverse transfer is easily inferred to be due to smaller magnetic islands merging to form larger ones via reconnection. We find that the scaling behaviour is similar between the 2D and the 3D cases, i.e., the magnetic energy evolves as $t^{-1}$, and the magnetic power spectrum follows a slope of $k^{-2}$. We show that on normalizing time by the magnetic reconnection timescale, the evolution curves of the magnetic field in systems with different Lundquist numbers collapse onto one another. Furthermore, transfer function plots show signatures of magnetic reconnection driving the inverse transfer. We also discuss the conserved quantities in the system and show that the behaviour of these quantities is similar between the 2D and 3D simulations, thus making the case that the dynamics in 3D could be approximately explained by what we understand in 2D. Lastly, we also conduct simulations where the magnetic field is subdominant to the flow. Here, too, we find an inverse transfer of magnetic energy in 3D. In these simulations, the magnetic energy evolves as $ t^{-1.4}$ and, interestingly, a dynamo effect is observed.