Comparison of Spectral Slopes of Magnetohydrodynamic and Hydrodynamic Turbulence and Measurements of Alignment Effects

We performed a series of high-resolution (up to 1024) direct numerical simulations of hydro and MHD turbulence. Our simulations correspond to the ”strong” MHD turbulence regime that cannot be treated perturbatively. We found that for simulations with normal viscosity the slopes for energy spectra of MHD are similar to ones in hydro, although slightly more shallower. However, for simulations with hyper viscosity 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. Non-locality of MHD turbulence casts doubt on numerical determination of the slopes with currently available (512–1024) numerical resolutions, including simulations with normal viscosity. We also measure various so-called alignment effects and discuss their influence on the turbulent cascade. Subject headings: MHD – turbulence – ISM: kinematics and dynamics