Comparison of spectral slopes of magnetohydrodynamic and hydrodynamic turbulence and measurements of alignment effects

TL;DR

This study compares spectral slopes of magnetohydrodynamic and hydrodynamic turbulence through high-resolution simulations, revealing differences in localness and the impact of viscosity types on turbulence characteristics.

Contribution

It demonstrates that MHD turbulence is less local than hydrodynamic turbulence, challenging existing models assuming local cascading, and discusses the influence of alignment effects on turbulence.

Findings

MHD spectra are slightly shallower than hydro spectra with normal viscosity.

Hyperviscosity affects hydro and MHD spectra differently, highlighting nonlocality in MHD.

Alignment effects influence the turbulent cascade and are analyzed in the study.

Abstract

We performed a series of high-resolution (up to 1024^3) direct numerical simulations of hydro and MHD strong turbulence. We found that for simulations with normal viscosity the slopes for spectra of MHD are similar, although slightly more shallower than for hydro simulations. However, for simulations with hyperviscosity the slopes were very different, for instance, the slopes for hydro simulations showed pronounced and well-defined bottleneck effect, while the MHD slopes were relatively much less affected. We believe that this is indicative of MHD strong turbulence being less local than Kolmogorov turbulence. This calls for revision of MHD strong turbulence models that assume local "as-in-hydro case" cascading. Nonlocality of MHD turbulence casts doubt on numerical determination of the slopes with currently available (512^3--1024^3) numerical resolutions, including simulations with normal viscosity. We also measure various so-called alignment effects and discuss their influence on the turbulent cascade.