398 research outputs found
Growth of Magnetic Fields Induced by Turbulent Motions
We present numerical simulations of driven magnetohydrodynamic (MHD)
turbulence with weak/moderate imposed magnetic fields. The main goal is to
clarify dynamics of magnetic field growth. We also investigate the effects of
the imposed magnetic fields on the MHD turbulence, including, as a limit, the
case of zero external field. Our findings are as follows. First, when we start
off simulations with weak mean magnetic field only (or with small scale random
field with zero imposed field), we observe that there is a stage at which
magnetic energy density grows linearly with time. Runs with different numerical
resolutions and/or different simulation parameters show consistent results for
the growth rate at the linear stage. Second, we find that, when the strength of
the external field increases, the equilibrium kinetic energy density drops by
roughly the product of the rms velocity and the strength of the external field.
The equilibrium magnetic energy density rises by roughly the same amount.
Third, when the external magnetic field is not very strong (say, less than ~0.2
times the rms velocity when measured in the units of Alfven speed), the
turbulence at large scales remains statistically isotropic, i.e. there is no
apparent global anisotropy of order B_0/v. We discuss implications of our
results on astrophysical fluids.Comment: 16 pages, 18 figures; ApJ, accepte
Numerical Studies of Weakly Stochastic Magnetic Reconnection
We study the effects of turbulence on magnetic reconnection using
three-dimensional numerical simulations. This is the first attempt to test a
model of fast magnetic reconnection proposed by Lazarian & Vishniac (1999),
which assumes the presence of weak, small-scale magnetic field structure near
the current sheet. This affects the rate of reconnection by reducing the
transverse scale for reconnection flows and by allowing many independent flux
reconnection events to occur simultaneously. We performed a number of
simulations to test the dependencies of the reconnection speed, defined as the
ratio of the inflow velocity to the Alfven speed, on the turbulence power, the
injection scale and resistivity. Our results show that turbulence significantly
affects the topology of magnetic field near the diffusion region and increases
the thickness of the outflow region. We confirm the predictions of the Lazarian
& Vishniac model. In particular, we report the growth of the reconnection speed
proportional to ~ V^2, where V is the amplitude of velocity at the injection
scale. It depends on the injection scale l as ~ (l/L)^(2/3), where L is the
size of the system, which is somewhat faster but still roughly consistent with
the theoretical expectations. We also show that for 3D reconnection the Ohmic
resistivity is important in the local reconnection events only, and the global
reconnection rate in the presence of turbulence does not depend on it.Comment: 8 pages, 8 figure
Gravitational Instability in Collisionless Cosmological Pancakes
The gravitational instability of cosmological pancakes composed of
collisionless dark matter in an Einstein-de Sitter universe is investigated
numerically to demonstrate that pancakes are unstable with respect to
fragmentation and the formation of filaments. A ``pancake'' is defined here as
the nonlinear outcome of the growth of a 1D, sinusoidal, plane-wave, adiabatic
density perturbation. We have used high resolution, 2D, N-body simulations by
the Particle-Mesh (PM) method to study the response of pancakes to perturbation
by either symmetric (density) or antisymmetric (bending or rippling) modes,
with corresponding wavevectors k_s and k_a transverse to the wavevector k_p of
the unperturbed pancake plane-wave. We consider dark matter which is initially
``cold'' (i.e. with no random thermal velocity in the initial conditions). We
also investigate the effect of a finite, random, isotropic, initial velocity
dispersion (i.e. initial thermal velocity) on the fate of pancake collapse and
instability. Pancakes are shown to be gravitationally unstable with respect to
all perturbations of wavelength l<l_p (where l_p= 2pi/k_p). These results are
in contradiction with the expectations of an approximate, thin-sheet energy
argument.Comment: To appear in the Astrophysical Journal (1997), accepted for
publication 10/10/96, single postscript file, 61 pages, 19 figure
The 21cm angular-power spectrum from the dark ages
At redshifts z >~ 30 neutral hydrogen gas absorbs CMB radiation at the 21cm
spin-flip frequency. In principle this is observable and a high-precision probe
of cosmology. We calculate the linear-theory angular power spectrum of this
signal and cross-correlation between redshifts on scales much larger than the
line width. In addition to the well known redshift-distortion and density
perturbation sources a full linear analysis gives additional contributions to
the power spectrum. On small scales there is a percent-level linear effect due
to perturbations in the 21cm optical depth, and perturbed recombination
modifies the gas temperature perturbation evolution (and hence spin temperature
and 21cm power spectrum). On large scales there are several post-Newtonian and
velocity effects; although negligible on small scales, these additional terms
can be significant at l <~ 100 and can be non-zero even when there is no
background signal. We also discuss the linear effect of reionization
re-scattering, which damps the entire spectrum and gives a very small
polarization signal on large scales. On small scales we also model the
significant non-linear effects of evolution and gravitational lensing. We
include full results for numerical calculation and also various approximate
analytic results for the power spectrum and evolution of small scale
perturbations.Comment: 29 pages; significant extensions including: self-absorption terms
(i.e. change to background radiation due to 21cm absorption); ionization
fraction perturbations; estimates of non-linear effects; approximate analytic
results; results for sharp redshift window functions. Code available at
http://camb.info/sources
Magnetic Field Structure and Stochastic Reconnection in a Partially Ionized Gas
We consider stochastic reconnection in a magnetized, partially ionized
medium. Stochastic reconnection is a generic effect, due to field line
wandering, in which the speed of reconnection is determined by the ability of
ejected plasma to diffuse away from the current sheet along magnetic field
lines, rather than by the details of current sheet structure. We consider the
limit of weak stochasticity, so that the mean magnetic field energy density is
greater than either the turbulent kinetic energy density or the energy density
associated with the fluctuating component of the field. We consider field line
stochasticity generated through a turbulent cascade, which leads us to consider
the effect of neutral drag on the turbulent cascade of energy. In a
collisionless plasma, neutral particle viscosity and ion-neutral drag will damp
mid-scale turbulent motions, but the power spectrum of the magnetic
perturbations extends below the viscous cutoff scale. We give a simple physical
picture of the magnetic field structure below this cutoff, consistent with
numerical experiments. We provide arguments for the reemergence of the
turbulent cascade well below the viscous cut-off scale and derive estimates for
field line diffusion on all scales. We note that this explains the persistence
of a single power law form for the turbulent power spectrum of the interstellar
medium, from scales of tens of parsecs down to thousands of kilometers. We find
that under typical conditions in the ISM stochastic reconnection speeds are
reduced by the presence of neutrals, but by no more than an order of magnitude.Comment: Astrophysical Journal in pres
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