Scaling properties of small-scale fluctuations in magnetohydrodynamic turbulence

TL;DR

This paper investigates the scaling properties of small-scale fluctuations in magnetohydrodynamic turbulence, revealing fundamental differences from hydrodynamic turbulence and implications for numerical simulation resolution.

Contribution

It demonstrates that the Kolmogorov self-similarity hypothesis does not apply to MHD turbulence and establishes new resolution requirements for numerical simulations.

Findings

Kolmogorov self-similarity hypothesis fails for MHD turbulence

Numerical resolution must decrease faster than dissipation scale with increasing Reynolds number

Scaling laws in MHD turbulence differ fundamentally from hydrodynamic turbulence

Abstract

Magnetohydrodynamic (MHD) turbulence in the majority of natural systems, including the interstellar medium, the solar corona, and the solar wind, has Reynolds numbers far exceeding the Reynolds numbers achievable in numerical experiments. Much attention is therefore drawn to the universal scaling properties of small-scale fluctuations, which can be reliably measured in the simulations and then extrapolated to astrophysical scales. However, in contrast with hydrodynamic turbulence, where the universal structure of the inertial and dissipation intervals is described by the Kolmogorov self-similarity, the scaling for MHD turbulence cannot be established based solely on dimensional arguments due to the presence of an intrinsic velocity scale -- the Alfven velocity. In this Letter, we demonstrate that the Kolmogorov first self-similarity hypothesis cannot be formulated for MHD turbulence in the same way it is formulated for the hydrodynamic case. Besides profound consequences for the analytical consideration, this also imposes stringent conditions on numerical studies of MHD turbulence. In contrast with the hydrodynamic case, the discretization scale in numerical simulations of MHD turbulence should decrease faster than the dissipation scale, in order for the simulations to remain resolved as the Reynolds number increases.