arXiv subjects

Andrey Beresnyak

Publications and source records attributed to Andrey Beresnyak.

22 records · Page 2Linked to original sources

arXiv2009

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

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.

astro-ph↗

arXiv2008

Wave Decay in MHD Turbulence

We present a model for nonlinear decay of the weak wave in three-dimensional incompressible magnetohydrodynamic (MHD) turbulence. We show that the decay rate is different for parallel and perpendicular waves. We provide a general formula for arbitrarily directed waves and discuss particular limiting cases known in the literature. We test our predictions with direct numerical simulations of wave decay in three-dimensional MHD turbulence, and discuss the influence of turbulent damping on the development of linear instabilities in the interstellar medium and on other important astrophysical processes.

astro-ph↗

arXiv2006

Cosmic Ray Scattering in Compressible Turbulence

We study the scattering of low-energy Cosmic Rays (CRs) in a turbulent, compressive MHD fluid. We show that compressible MHD modes -- fast or slow waves with wave lengths smaller than CR mean free paths induce cyclotron instability in CRs. The instability feeds the new small-scale Alfvenic wave component with wave vectors mostly along magnetic field, which is not a part of the MHD turbulence cascade. This new component gives feedback on the instability through decreasing the CR mean free path. We show that the ambient turbulence fully suppresses the instability at large scales, while wave steepening constrains the amplitude of the waves at small scales. We provide the energy spectrum of the plane-parallel Alfvenic component and calculate mean free paths of CRs as a function of their energy. We find that for the typical parameters of turbulence in the interstellar medium and in the intercluster medium the new Alfvenic component provides the scattering of the low energy CRs that exceeds the direct resonance scattering by MHD modes. This solves the problem of insufficient scattering of low-energy CRs in the turbulent interstellar or intracluster medium that was reported in the literature.

astro-ph↗

arXiv2006

Polarization intermittency in MHD and its influence on turbulent cascade

Goldreich-Sridhar model of incompressible turbulence provides an elegant approach to describing strong MHD turbulence. It relies on the fact that interacting Alfvenic waves are independent and have random polarization. However, in case of strong interaction a spontaneous local assymetry can arise. We used direct numerical simulations to show that polarization alignment occurs and it grows larger at smaller scales. This effect would lead to a shallower spectrum and stronger anisotropy. Even small changes in these two properties will have important astrophysical consequencies, e.g. for the cosmic ray physics.

astro-ph↗