334 research outputs found
Numerical study of Cosmic Ray Diffusion in MHD turbulence
We study diffusion of Cosmic Rays (CRs) in turbulent magnetic fields using
test particle simulations. Electromagnetic fields are produced in direct
numerical MHD simulations of turbulence and used as an input for particle
tracing, particle feedback on turbulence being ignored. Statistical transport
coefficients from the test particle runs are compared with earlier analytical
predictions. We find qualitative correspondence between them in various aspects
of CR diffusion. In the incompressible case, that we consider in this paper,
the dominant scattering mechanism occurs to be the non-resonant mirror
interactions with the slow-mode perturbations. Perpendicular transport roughly
agrees with being produced by magnetic field wandering.Comment: 8 pages, 8 figures, ApJ sub
Non-locality of Hydrodynamic and Magnetohydrodynamic Turbulence
We compare non-locality of interactions between different scales in
hydrodynamic (HD) turbulence and magnetohydrodynamic (MHD) turbulence in a
strongly magnetized medium. We use 3-dimensional incompressible direct
numerical simulations to evaluate non-locality of interactions. Our results
show that non-locality in MHD turbulence is much more pronounced than that in
HD turbulence. Roughly speaking, non-local interactions count for more than
10\% of total interactions in our MHD simulation on a grid of points.
However, there is no evidence that non-local interactions are important in our
HD simulation with the same numerical resolution. We briefly discuss how
non-locality affects energy spectrum.Comment: 6 pages, 5 figure
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.Comment: 10 pages, 6 figures, extended version, ApJ accepte
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