649 research outputs found
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.Comment: 7 pages, 5 figures, to appear in ApJ 67
Radio-wave propagation through a medium containing electron-density fluctuations described by an anisotropic Goldreich-Sridhar spectrum
We study the propagation of radio waves through a medium possessing density
fluctuations that are elongated along the ambient magnetic field and described
by an anisotropic Goldreich-Sridhar power spectrum. We derive general formulas
for the wave phase structure function, visibility, angular broadening,
diffraction-pattern length scales, and scintillation time scale for arbitrary
distributions of turbulence along the line of sight, and specialize these
formulas to idealized cases.Comment: 25 pages, 3 figures, submitted to Ap
Scaling law of the plasma turbulence with non conservative fluxes
It is shown that in the presence of anisotropic kinetic dissipation existence
of scale invariant power law spectrum of plasma turbulence is possible.
Obtained scale invariant spectrum is not associated with the constant flux of
any physical quantity. Application of the model to the high frequency part of
the solar wind turbulence is discussed.Comment: Phys Rev E, accepte
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
Dust Dynamics in Compressible MHD Turbulence
We calculate the relative grain-grain motions arising from interstellar
magnetohydrodynamic (MHD) turbulence. The MHD turbulence includes both fluid
motions and magnetic fluctuations. While the fluid motions accelerate grains
through hydro-drag, the electromagnetic fluctuations accelerate grains through
resonant interactions. We consider both incompressive (Alfv\'{e}n) and
compressive (fast and slow) MHD modes and use descriptions of MHD turbulence
obtained in Cho & Lazarian (2002). Calculations of grain relative motion are
made for realistic grain charging and interstellar turbulence that is
consistent with the velocity dispersions observed in diffuse gas, including
cutoff of the turbulence from various damping processes. We show that fast
modes dominate grain acceleration, and can drive grains to supersonic
velocities. Grains are also scattered by gyroresonance interactions, but the
scattering is less important than acceleration for grains moving with
sub-Alfv\'{e}nic velocities. Since the grains are preferentially accelerated
with large pitch angles, the supersonic grains will be aligned with long axes
perpendicular to the magnetic field. We compare grain velocities arising from
MHD turbulence with those arising from photoelectric emission, radiation
pressure and H thrust. We show that for typical interstellar conditions
turbulence should prevent these mechanisms from segregating small and large
grains. Finally, gyroresonant acceleration is bound to preaccelerate grains
that are further accelerated in shocks. Grain-grain collisions in the shock may
then contribute to the overabundance of refractory elements in the composition
of galactic cosmic rays.Comment: 15 pages, 17 figure
Spectral energy dynamics in magnetohydrodynamic turbulence
Spectral direct numerical simulations of incompressible MHD turbulence at a
resolution of up to collocation points are presented for a
statistically isotropic system as well as for a setup with an imposed strong
mean magnetic field. The spectra of residual energy,
, and total energy,
, are observed to scale self-similarly in
the inertial range as ,
(isotropic case) and ,
(anisotropic case, perpendicular to the mean
field direction). A model of dynamic equilibrium between kinetic and magnetic
energy, based on the corresponding evolution equations of the eddy-damped
quasi-normal Markovian (EDQNM) closure approximation, explains the findings.
The assumed interplay of turbulent dynamo and Alfv\'en effect yields
which is confirmed by the simulations.Comment: accepted for publication by PR
Particle acceleration by slow modes in strong compressible MHD turbulence, with application to solar flares
Energetic particles that undergo strong pitch-angle scattering and diffuse
through a plasma containing strong compressible MHD turbulence undergo
diffusion in momentum space with diffusion coefficient Dp. In this paper, the
contribution of slow modes to Dp is calculated assuming the rms turbulent
velocity is of order the Alfven speed. The energy spectrum of accelerated
particles is derived assuming slow modes make the dominant contribution to Dp,
taking into account Coulomb losses and particle escape from the acceleration
region with an energy-independent escape time. The results are applied to solar
flares.Comment: 19 pages, 3 figures, accepted for publication in Ap
Particle Heating by Alfvenic Turbulence in Hot Accretion Flows
Recent work on Alfvenic turbulence by Goldreich & Sridhar (1995; GS) suggests
that the energy cascades almost entirely perpendicular to the local magnetic
field. As a result, the cyclotron resonance is unimportant in dissipating the
turbulent energy. Motivated by the GS cascade, we calculate the linear
collisionless dissipation of Alfven waves with frequencies much less than the
proton cyclotron frequency, but with perpendicular wavelengths of order the
Larmor radius of thermal protons. In plasmas appropriate to hot accretion flows
(proton temperature much greater than electron temperature) the dissipated
Alfven wave energy primarily heats the protons. For a plasma with \beta \lsim
5, however, where is the ratio of the gas pressure to the magnetic
pressure, the MHD assumptions utilized in the GS analysis break down before
most of the energy in Alfven waves is dissipated; how the cascade then proceeds
is unclear. Hot accretion flows, such as advection dominated accretion flows
(ADAFs), are expected to contain significant levels of MHD turbulence. This
work suggests that, for \beta \gsim 5, the Alfvenic component of such
turbulence primarily heats the protons. Significant proton heating is required
for the viability of ADAF models. We contrast our results on particle heating
in ADAFs with recent work by Bisnovatyi-Kogan & Lovelace (1997).Comment: 35 pages (Latex), 4 Figures. Submitted to Ap
The Milky Way's Kiloparsec Scale Wind: A Hybrid Cosmic-Ray and Thermally Driven Outflow
We apply a wind model, driven by combined cosmic-ray and thermal-gas
pressure, to the Milky Way, and show that the observed Galactic diffuse soft
X-ray emission can be better explained by a wind than by previous static gas
models. We find that cosmic-ray pressure is essential to driving the observed
wind. Having thus defined a "best-fit" model for a Galactic wind, we explore
variations in the base parameters and show how the wind's properties vary with
changes in gas pressure, cosmic-ray pressure and density. We demonstrate the
importance of cosmic rays in launching winds, and the effect cosmic rays have
on wind dynamics. In addition, this model adds support to the hypothesis of
Breitschwerdt and collaborators that such a wind may help explain the
relatively small gradient observed in gamma-ray emission as a function of
galactocentric radius.Comment: 14 pages, 11 figures; Accepted to Ap
Measurement of the cross section with the CMD-3 detector at the VEPP-2000 collider
The process has been studied in the
center-of-mass energy range from 1500 to 2000\,MeV using a data sample of 23
pb collected with the CMD-3 detector at the VEPP-2000 collider.
Using about 24000 selected events, the cross
section has been measured with a systematic uncertainty decreasing from 11.7\%
at 1500-1600\,MeV to 6.1\% above 1800\,MeV. A preliminary study of
production dynamics has been performed
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