636 research outputs found
Weak compressible magnetohydrodynamic turbulence in the solar corona
This Letter presents a calculation of the power spectra of weakly turbulent
Alfven waves and fast magnetosonic waves ("fast waves") in low-beta plasmas. It
is shown that three-wave interactions transfer energy to high-frequency fast
waves and, to a lesser extent, high-frequency Alfven waves. MHD turbulence is
thus a promising mechanism for producing the high-frequency waves needed to
explain the anisotropic heating of minor ions in the solar corona.Comment: 4 pages, 3 figures, accepted, Phys. Rev. Let
Weakly Turbulent MHD Waves in Compressible Low-Beta Plasmas
In this Letter, weak turbulence theory is used to investigate interactions
among Alfven waves and fast and slow magnetosonic waves in collisionless
low-beta plasmas. The wave kinetic equations are derived from the equations of
magnetohydrodynamics, and extra terms are then added to model collisionless
damping. These equations are used to provide a quantitative description of a
variety of nonlinear processes, including "parallel" and "perpendicular" energy
cascade, energy transfer between wave types, "phase mixing," and the generation
of back-scattered Alfven waves.Comment: Accepted, Physical Review Letter
Shell-models of RMHD turbulence and the heating of solar coronal loops
A simplified non-linear numerical model for the development of incompressible
magnetohydrodynamics (MHD) in the presence of a strong magnetic field B0 and
stratification, nicknamed Shell-Atm, is presented. In planes orthogonal to the
mean field, the non-linear incompressible dynamics is replaced by 2D
shell-models for the complex variables u and b, allowing one to reach large
Reynolds numbers while at the same time carrying out sufficiently long time
integrations to obtain a good statistics at moderate computational cost. The
shell-models of different planes are coupled by Alfven waves propagating along
B0. The model may be applied to open or closed magnetic field configurations
where the axial field dominates and the plasma pressure is low; here we apply
it to the specific case of a magnetic loop of the solar corona heated via
turbulence driven by photospheric motions, and we use statistics for its
analysis. The Alfven waves interact non-linearly and form turbulent spectra in
the directions perpendicular and, via propagation, also parallel to the mean
field. A heating function is obtained, and is shown to be intermittent; the
average heating is consistent with values required for sustaining a hot corona,
and is proportional to the aspect ratio of the loop to the power -1.5;
characteristic properties of heating events are distributed as power-laws.
Cross-correlations show a delay of dissipation compared to energy content.Comment: 12 pages, 16 figures, accepted for publication in 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
Particle Acceleration by MHD Turbulence
Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call
for revisions in the picture of particle acceleration. We make use of the
recently established scaling of slow and fast MHD modes in strong and weak MHD
turbulence to provide a systematic study of particle acceleration in magnetic
pressure (low-) and gaseous pressure (high-) dominated plasmas.
We consider the acceleration by large scale compressions in both slow and fast
particle diffusion limits. We compare the results with the acceleration rate
that arises from resonance scattering and Transit-Time Damping (TTD).
We establish that fast modes accelerate particles more efficiently than slow
modes. We find that particle acceleration by pitch-angle scattering and TTD
dominates acceleration by slow or fast modes when the spatial diffusion rate is
small. When the rate of spatial diffusion of particles is high, we establish an
enhancement of the efficiency of particle acceleration by slow and fast modes
in weak turbulence. We show that highly supersonic turbulence is an efficient
agent for particle acceleration. We find that even incompressible turbulence
can accelerate particles on the scales comparable with the particle mean free
path.Comment: 17 pages, 4 figures, accepted by ApJ; Some sections re-arrange
Compressible Sub-Alfvenic MHD turbulence in Low-beta Plasmas
We present a model for compressible sub-Alfvenic isothermal
magnetohydrodynamic (MHD) turbulence in low-beta plasmas and numerically test
it. We separate MHD fluctuations into 3 distinct families - Alfven, slow, and
fast modes. We find that, production of slow and fast modes by Alfvenic
turbulence is suppressed. As a result, Alfven modes in compressible regime
exhibit scalings and anisotropy similar to those in incompressible regime. Slow
modes passively mimic Alfven modes. However, fast modes show isotropy and a
scaling similar to acoustic turbulence.Comment: 4 pages, 8 figures, Phys. Rev. Lett., in pres
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
Coronal heating distribution due to low-frequency wave-driven turbulence
The heating of the lower solar corona is examined using numerical simulations
and theoretical models of magnetohydrodynamic turbulence in open magnetic
regions. A turbulent energy cascade to small length scales perpendicular to the
mean magnetic field can be sustained by driving with low-frequency Alfven waves
reflected from mean density and magnetic field gradients. This mechanism
deposits energy efficiently in the lower corona, and we show that the spatial
distribution of the heating is determined by the mean density through the
Alfven speed profile. This provides a robust heating mechanism that can explain
observed high coronal temperatures and accounts for the significant heating
(per unit volume) distribution below two solar radius needed in models of the
origin of the solar wind. The obtained heating per unit mass on the other hand
is much more extended indicating that the heating on a per particle basis
persists throughout all the lower coronal region considered here.Comment: 19 pages, 5 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
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