22,583 research outputs found
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
Damping of MHD turbulence in partially ionized plasma: implications for cosmic ray propagation
We study the damping from neutral-ion collisions of both incompressible and
compressible magnetohydrodynamic (MHD) turbulence in partially ionized medium.
We start from the linear analysis of MHD waves applying both single-fluid and
two-fluid treatments. The damping rates derived from the linear analysis are
then used in determining the damping scales of MHD turbulence. The physical
connection between the damping scale of MHD turbulence and cutoff boundary of
linear MHD waves is investigated. Our analytical results are shown to be
applicable in a variety of partially ionized interstellar medium (ISM) phases
and solar chromosphere. As a significant astrophysical utility, we introduce
damping effects to propagation of cosmic rays in partially ionized ISM. The
important role of turbulence damping in both transit-time damping and
gyroresonance is identified.Comment: 29 pages, 16 figure
Dynamic Alignment and Exact Scaling Laws in MHD Turbulence
Magnetohydrodynamic (MHD) turbulence is pervasive in astrophysical systems.
Recent high-resolution numerical simulations suggest that the energy spectrum
of strong incompressible MHD turbulence is . So far, there has been no phenomenological theory that
simultaneously explains this spectrum and satisfies the exact analytic
relations for MHD turbulence due to Politano & Pouquet. Indeed, the
Politano-Pouquet relations are often invoked to suggest that the spectrum of
MHD turbulence instead has the Kolmogorov scaling -5/3. Using geometrical
arguments and numerical tests, here we analyze this seeming contradiction and
demonstrate that the -3/2 scaling and the Politano-Pouquet relations are
reconciled by the phenomenon of scale-dependent dynamic alignment that was
recently discovered in MHD turbulence.Comment: Published versio
Scaling, Intermittency and Decay of MHD Turbulence
We discuss a few recent developments that are important for understanding of
MHD turbulence.
First, MHD turbulence is not so messy as it is usually believed. In fact, the
notion of strong non-linear coupling of compressible and incompressible motions
along MHD cascade is not tenable. Alfven, slow and fast modes of MHD turbulence
follow their own cascades and exhibit degrees of anisotropy consistent with
theoretical expectations. Second, the fast decay of turbulence is not related
to the compressibility of fluid. Rates of decay of compressible and
incompressible motions are very similar. Third, viscosity by neutrals does not
suppress MHD turbulence in a partially ionized gas. Instead, MHD turbulence
develops magnetic cascade at scales below the scale at which neutrals damp
ordinary hydrodynamic motions. Forth, density statistics does not exhibit the
universality that the velocity and magnetic field do. For instance, at small
Mach numbers the density is anisotropic, but it gets isotropic at high Mach
numbers. Fifth, the intermittency of magnetic field and velocity are different.
Both depend on whether the measurements are done in local system of reference
oriented along the local magnetic field or in the global system of reference
related to the mean magnetic field.Comment: 12 pages, Invited Review, Workshop on Theoretical Plasma Physics,
Trieste, Italy, 5-16 Jul
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