1,167 research outputs found
Distribution of Magnetic Discontinuities in the Solar Wind and in MHD Turbulence
The statistical properties of magnetic discontinuities in the solar wind are
investigated by measuring fluctuations in the magnetic field direction, given
by the rotation Delta theta that the magnetic field vector undergoes during
time interval Delta t. We show that the probability density function for
rotations, P(Delta theta), can be described by a simple model in which the
magnetic field vector rotates with a relative increment (Delta B)/B that is
lognormally distributed. We find that the probability density function of
increments, P((Delta B)/B), has a remarkable scaling property: the normalized
variable x=[(Delta B)/B]*[(Delta t)/(Delta t_0)]^-a has a universal lognormal
distribution for all time intervals Delta t. We then compare measurements from
the solar wind with those from direct numerical simulations of
magnetohydrodynamic (MHD) turbulence. We find good agreement for P(Delta theta)
obtained in the two cases when the magnetic guide-field to fluctuations ratio
B_0/b_rms is chosen accordingly. However, the scale invariance of P((Delta
B)/B) is broken in the MHD simulations with relatively limited inertial
interval, which causes P(Delta theta) to scale with measurement interval
differently than in the solar wind.Comment: To appear in Astrophysical Journal Letter
Temporal Intermittency of Energy Dissipation in Magnetohydrodynamic Turbulence
Energy dissipation in magnetohydrodynamic (MHD) turbulence is known to be
highly intermittent in space, being concentrated in sheet-like coherent
structures. Much less is known about intermittency in time, another fundamental
aspect of turbulence which has great importance for observations of solar
flares and other space/astrophysical phenomena. In this Letter, we investigate
the temporal intermittency of energy dissipation in numerical simulations of
MHD turbulence. We consider four-dimensional spatiotemporal structures, "flare
events", responsible for a large fraction of the energy dissipation. We find
that although the flare events are often highly complex, they exhibit robust
power-law distributions and scaling relations. We find that the probability
distribution of dissipated energy has a power law index close to -1.75, similar
to observations of solar flares, indicating that intense dissipative events
dominate the heating of the system. We also discuss the temporal asymmetry of
flare events as a signature of the turbulent cascade.Comment: To appear in Physical Review Letters. 6 pages, 4 figure
Intermittency of Energy Dissipation in Alfvenic Turbulence
We investigate the intermittency of energy dissipation in Alfvenic turbulence
by considering the statistics of the coarse-grained energy dissipation rate,
using direct measurements from numerical simulations of magnetohydrodynamic
turbulence and surrogate measurements from the solar wind. We compare the
results to the predictions of the log-normal and log-Poisson random cascade
models. We find that, to a very good approximation, the log-normal model
describes the probability density function for the energy dissipation over a
broad range of scales, but does not accurately describe the scaling exponents
of the moments. The log-Poisson model better describes the scaling exponents of
the moments, while the comparison with the probability density function is not
straightforward.Comment: To appear in MNRAS Letters. 5 pages, 4 figure
Particle energization in relativistic plasma turbulence: solenoidal versus compressive driving
Many high-energy astrophysical systems contain magnetized collisionless
plasmas with relativistic particles, in which turbulence can be driven by an
arbitrary mixture of solenoidal and compressive motions. For example,
turbulence in hot accretion flows may be driven solenoidally by the
magnetorotational instability or compressively by spiral shock waves. It is
important to understand the role of the driving mechanism on kinetic turbulence
and the associated particle energization. In this work, we compare
particle-in-cell simulations of solenoidally driven turbulence with similar
simulations of compressively driven turbulence. We focus on plasma that has an
initial beta of unity, relativistically hot electrons, and varying ion
temperature. Apart from strong large-scale density fluctuations in the
compressive case, the turbulence statistics are similar for both drives, and
the bulk plasma is described reasonably well by an isothermal equation of
state. We find that nonthermal particle acceleration is more efficient when
turbulence is driven compressively. In the case of relativistically hot ions,
both driving mechanisms ultimately lead to similar power-law particle energy
distributions, but over a different duration. In the case of non-relativistic
ions, there is significant nonthermal particle acceleration only for
compressive driving. Additionally, we find that the electron-to-ion heating
ratio is less than unity for both drives, but takes a smaller value for
compressive driving. We demonstrate that this additional ion energization is
associated with the collisionless damping of large-scale compressive modes via
perpendicular electric fields.Comment: 29 pages, 28 figures, accepted for publication in Ap
Statistical Analysis of Current Sheets in Three-Dimensional Magnetohydrodynamic Turbulence
We develop a framework for studying the statistical properties of current
sheets in numerical simulations of 3D magnetohydrodynamic (MHD) turbulence. We
describe an algorithm that identifies current sheets in a simulation snapshot
and then determines their geometrical properties (including length, width, and
thickness) and intensities (peak current density and total energy dissipation
rate). We then apply this procedure to simulations of reduced MHD turbulence
and perform a statistical analysis on the obtained population of current
sheets. We evaluate the role of reconnection by separately studying the
populations of current sheets which contain magnetic X-points and those which
do not. We find that the statistical properties of the two populations are
different in general. We compare the scaling of these properties to
phenomenological predictions obtained for the inertial range of MHD turbulence.
Finally, we test whether the reconnecting current sheets are consistent with
the Sweet-Parker model.Comment: 19 pages, 19 figure
Spectral breaks of Alfvenic turbulence in a collisionless plasma
Recent observations reveal that magnetic turbulence in the nearly
colisionless solar wind plasma extends to scales smaller than the plasma
microscales, such as ion gyroradius and ion inertial length. Measured breaks in
the spectra of magnetic and density fluctuations at high frequencies are
thought to be related to the transition from large-scale hydromagnetic to
small-scale kinetic turbulence. The scales of such transitions and the
responsible physical mechanisms are not well understood however. In the present
work we emphasize the crucial role of the plasma parameters in the transition
to kinetic turbulence, such as the ion and electron plasma beta, the electron
to ion temperature ratio, the degree of obliquity of turbulent fluctuations. We
then propose an explanation for the spectral breaks reported in recent
observations.Comment: 9 pages, 7 figures. A few typos found in the published version are
correcte
System-size convergence of nonthermal particle acceleration in relativistic plasma turbulence
We apply collisionless particle-in-cell simulations of relativistic pair
plasmas to explore whether driven turbulence is a viable high-energy
astrophysical particle accelerator. We characterize nonthermal particle
distributions for varying system sizes up to , where
is the driving scale and is the initial characteristic
Larmor radius. We show that turbulent particle acceleration produces power-law
energy distributions that, when compared at a fixed number of large-scale
dynamical times, slowly steepen with increasing system size. We demonstrate,
however, that convergence is obtained by comparing the distributions at
different times that increase with system size (approximately logarithmically).
We suggest that the system-size dependence arises from the time required for
particles to reach the highest accessible energies via Fermi acceleration. The
converged power-law index of the energy distribution, for
magnetization , makes turbulence a possible explanation for
nonthermal spectra observed in systems such as the Crab nebula.Comment: 7 pages, 4 figures, submitted to Astrophysical Journal Letter
Electron and ion energization in relativistic plasma turbulence
Electron and ion energization (i.e., heating and nonthermal acceleration) is
a fundamental, but poorly understood, outcome of plasma turbulence. In this
work, we present new results on this topic from particle-in-cell simulations of
driven turbulence in collisionless, relativistic electron-ion plasma. We focus
on temperatures such that ions (protons) are sub-relativistic and electrons are
ultra-relativistic, a regime relevant for high-energy astrophysical systems
such as hot accretion flows onto black holes. We find that ions tend to be
preferentially heated, gaining up to an order of magnitude more energy than
electrons, and propose a simple empirical formula to describe the electron-ion
energy partition as a function of the ratio of electron-to-ion gyroradii (which
in turn is a function of initial temperatures and plasma beta). We also find
that while efficient nonthermal particle acceleration occurs for both species
in the ultra-relativistic regime, nonthermal electron populations are
diminished with decreasing temperature whereas nonthermal ion populations are
essentially unchanged. These results have implications for modeling and
interpreting observations of hot accretion flows.Comment: 6 pages, 4 figures, accepted for publication in Physical Review
Letter
Numerical investigation of kinetic turbulence in relativistic pair plasmas I: Turbulence statistics
We describe results from particle-in-cell simulations of driven turbulence in
collisionless, magnetized, relativistic pair plasma. This physical regime
provides a simple setting for investigating the basic properties of kinetic
turbulence and is relevant for high-energy astrophysical systems such as pulsar
wind nebulae and astrophysical jets. In this paper, we investigate the
statistics of turbulent fluctuations in simulations on lattices of up to
cells and containing up to particles. Due to the
absence of a cooling mechanism in our simulations, turbulent energy dissipation
reduces the magnetization parameter to order unity within a few dynamical
times, causing turbulent motions to become sub-relativistic. In the developed
stage, our results agree with predictions from magnetohydrodynamic turbulence
phenomenology at inertial-range scales, including a power-law magnetic energy
spectrum with index near , scale-dependent anisotropy of fluctuations
described by critical balance, log-normal distributions for particle density
and internal energy density (related by a adiabatic index, as predicted
for an ultra-relativistic ideal gas), and the presence of intermittency. We
also present possible signatures of a kinetic cascade by measuring power-law
spectra for the magnetic, electric, and density fluctuations at sub-Larmor
scales.Comment: 24 pages, 26 figures, submitted for publicatio
Energy dynamics and current sheet structure in fluid and kinetic simulations of decaying magnetohydrodynamic turbulence
Simulations of decaying magnetohydrodynamic (MHD) turbulence are performed
with a fluid and a kinetic code. The initial condition is an ensemble of
long-wavelength, counter-propagating, shear-Alfv\'{e}n waves, which interact
and rapidly generate strong MHD turbulence. The total energy is conserved and
the rate of turbulent energy decay is very similar in both codes, although the
fluid code has numerical dissipation whereas the kinetic code has kinetic
dissipation. The inertial range power spectrum index is similar in both the
codes. The fluid code shows a perpendicular wavenumber spectral slope of
. The kinetic code shows a spectral slope of
for smaller simulation domain, and for
larger domain. We estimate that collisionless damping mechanisms in the kinetic
code can account for the dissipation of the observed nonlinear energy cascade.
Current sheets are geometrically characterized. Their lengths and widths are in
good agreement between the two codes. The length scales linearly with the
driving scale of the turbulence. In the fluid code, their thickness is
determined by the grid resolution as there is no explicit diffusivity. In the
kinetic code, their thickness is very close to the skin-depth, irrespective of
the grid resolution. This work shows that kinetic codes can reproduce the MHD
inertial range dynamics at large scales, while at the same time capturing
important kinetic physics at small scales
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