101 research outputs found
Magnetic Helicity Conservation and Inverse Energy Cascade in Electron Magnetohydrodynamic Wave Packets
Electron magnetohydrodynamics (EMHD) provides a fluid-like description of
small-scale magnetized plasmas. An EMHD wave (also known as whistler wave)
propagates along magnetic field lines. The direction of propagation can be
either parallel or anti-parallel to the magnetic field lines. We numerically
study propagation of 3-dimensional (3D) EMHD wave packets moving in one
direction. We obtain two major results: 1. Unlike its magnetohydrodynamic (MHD)
counterpart, an EMHD wave packet is dispersive. Because of this, EMHD wave
packets traveling in one direction create opposite traveling wave packets via
self-interaction and cascade energy to smaller scales. 2. EMHD wave packets
traveling in one direction clearly exhibit inverse energy cascade. We find that
the latter is due to conservation of magnetic helicity. We compare inverse
energy cascade in 3D EMHD turbulence and 2-dimensional (2D) hydrodynamic
turbulence.Comment: Phys. Rev. Lett., accepted (4pages, 4 figures
Turbulence and Particle Heating in Advection-Dominated Accretion Flows
We extend and reconcile recent work on turbulence and particle heating in
advection-dominated accretion flows. For approximately equipartition magnetic
fields, the turbulence primarily heats the electrons. For weaker magnetic
fields, the protons are primarily heated. The division between electron and
proton heating occurs between and (where
is the ratio of the gas to the magnetic pressure), depending on unknown
details of how Alfv\'en waves are converted into whistlers on scales of the
proton Larmor radius. We also discuss the possibility that magnetic
reconnection could be a significant source of electron heating.Comment: 17 pages (Latex), incl. 2 Figures; submitted to Ap
Electric fields in plasmas under pulsed currents
Electric fields in a plasma that conducts a high-current pulse are measured
as a function of time and space. The experiment is performed using a coaxial
configuration, in which a current rising to 160 kA in 100 ns is conducted
through a plasma that prefills the region between two coaxial electrodes. The
electric field is determined using laser spectroscopy and line-shape analysis.
Plasma doping allows for 3D spatially resolved measurements. The measured peak
magnitude and propagation velocity of the electric field is found to match
those of the Hall electric field, inferred from the magnetic-field front
propagation measured previously.Comment: 13 pages, 13 figures, submitted to PR
Generalised relativistic Ohm's laws, extended gauge transformations and magnetic linking
Generalisations of the relativistic ideal Ohm's law are presented that
include specific dynamical features of the current carrying particles in a
plasma. Cases of interest for space and laboratory plasmas are identified where
these generalisations allow for the definition of generalised electromagnetic
fields that transform under a Lorentz boost in the same way as the real
electromagnetic fields and that obey the same set of homogeneous Maxwell's
equations
On the Possibility of Development of the Explosion Instability in a Two-Component Gravitating System
We obtain an expression for the energy of the density wave propagating in a
multicomponent gravitating medium in the form well known from electrodynamics.
Using the above, the possibility of "triple production" of the quasi-particles,
or waves, with their energies summing up to zero, in a non-equilibrium medium
is demonstrated. That kind of resonance wave interaction is shown to result in
the development of an explosion instability. By the method developed in plasma
physics, the characteristic time of the instability is evaluated.Comment: 15 pages, 3 figures, accepted for publication (JETP
Density-shear instability in electron magneto-hydrodynamics
We discuss a novel instability in inertia-less electron magneto-hydrodynamics (EMHD), which arises from a combination of electron velocity shear and electron density gradients. The unstable modes have a lengthscale longer than the transverse density scale, and a growth-rate of the order of the inverse Hall timescale. We suggest that this density-shear instability may be of importance in magnetic reconnection regions on scales smaller than the ion skin depth, and in neutron star crusts. We demonstrate that the so-called Hall drift instability, previously argued to be relevant in neutron star crusts, is a resistive tearing instability rather than an instability of the Hall term itself. We argue that the density-shear instability is of greater significance in neutron stars than the tearing instability, because it generally has a faster growth-rate and is less sensitive to geometry and boundary conditions. We prove that, for uniform electron density, EMHD is "at least as stable" as regular, incompressible MHD, in the sense that any field configuration that is stable in MHD is also stable in EMHD. We present a connection between the density-shear instability in EMHD and the magneto-buoyancy instability in anelastic MHD
Kolmogorov's law for two-dimensional electron-magnetohydrodynamic turbulence
The analogue of the Kolmogorov's four-fifths law is derived for
two-dimensional, homogeneous, isotropic EMHD turbulence in the energy cascade
inertial range. Direct numerical simulations for the freely decaying case show
that this relation holds true for different values of the adimensional electron
inertial length scale, . The energy spectrum is found to be close to the
expected Kolmogorov spectrum.Comment: 9 pages RevTeX, 3 PostScript figure
Radiative Efficiency of Collisionless Accretion
Radiative efficiency of a slowly accreting black hole is estimated using a
two-temperature model of accretion. The radiative efficiency depends on the
magnetic field strength near the Schwarzschild radius. For weak magnetic fields
(magnetic energy=equipartition/1000), the low efficiency 0.0001 assumed in some
theoretical models might be achieved. For stronger fields, a significant
fraction of viscous heat is dissipated by electrons and radiated away resulting
in a larger efficiency. At equipartition magnetic fields, we estimate
efficiency = of order 10%.Comment: 12 pages, Latex, Submitted to Ap
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
Propagation of Electron Magnetohydrodynamic structures in a 2-D inhomogeneous plasma
The fully three dimensional governing equations in the electron
magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density is
obtained. These equations in the two dimensional (2-D) limit can be cast in
terms of the evolution of two coupled scalar fields. The nonlinear simulations
for the two dimensional case are carried out to understand the propagation of
EMHD magnetic structures in the presence of inhomogeneity. A novel effect
related to trapping of dipolar magnetic structures in the high density plasma
region in the EMHD regime is observed. The interpretation of this phenomena as
well as its relevance to the problem of hot spot generation in the context of
fast ignition is presented
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