1,485 research outputs found
Optically pumped resonance magnetometer for determining vectoral components in a spatial coordinate system Patent
Design and development of optically pumped resonance magnetometer for determining vectoral components in spatial coordinate syste
Edge of Chaos and Genesis of Turbulence
The edge of chaos is analyzed in a spatially extended system, modeled by the
regularized long-wave equation, prior to the transition to permanent
spatiotemporal chaos. In the presence of coexisting attractors, a chaotic
saddle is born at the basin boundary due to a smooth-fractal metamorphosis. As
a control parameter is varied, the chaotic transient evolves to well-developed
transient turbulence via a cascade of fractal-fractal metamorphoses. The edge
state responsible for the edge of chaos and the genesis of turbulence is an
unstable travelling wave in the laboratory frame, corresponding to a saddle
point lying at the basin boundary in the Fourier space
On-off intermittency and amplitude-phase synchronization in Keplerian shear flows
We study the development of coherent structures in local simulations of the
magnetorotational instability in accretion discs in regimes of on-off
intermittency. In a previous paper [Chian et al., Phys. Rev. Lett. 104, 254102
(2010)], we have shown that the laminar and bursty states due to the on-off
spatiotemporal intermittency in a one-dimensional model of nonlinear waves
correspond, respectively, to nonattracting coherent structures with higher and
lower degrees of amplitude-phase synchronization. In this paper we extend these
results to a three-dimensional model of magnetized Keplerian shear flows.
Keeping the kinetic Reynolds number and the magnetic Prandtl number fixed, we
investigate two different intermittent regimes by varying the plasma beta
parameter. The first regime is characterized by turbulent patterns interrupted
by the recurrent emergence of a large-scale coherent structure known as
two-channel flow, where the state of the system can be described by a single
Fourier mode. The second regime is dominated by the turbulence with sporadic
emergence of coherent structures with shapes that are reminiscent of a
perturbed channel flow. By computing the Fourier power and phase spectral
entropies in three-dimensions, we show that the large-scale coherent structures
are characterized by a high degree of amplitude-phase synchronization.Comment: 17 pages, 10 figure
A novel type of intermittency in a nonlinear dynamo in a compressible flow
The transition to intermittent mean--field dynamos is studied using numerical
simulations of isotropic magnetohydrodynamic turbulence driven by a helical
flow. The low-Prandtl number regime is investigated by keeping the kinematic
viscosity fixed while the magnetic diffusivity is varied. Just below the
critical parameter value for the onset of dynamo action, a transient
mean--field with low magnetic energy is observed. After the transition to a
sustained dynamo, the system is shown to evolve through different types of
intermittency until a large--scale coherent field with small--scale turbulent
fluctuations is formed. Prior to this coherent field stage, a new type of
intermittency is detected, where the magnetic field randomly alternates between
phases of coherent and incoherent large--scale spatial structures. The
relevance of these findings to the understanding of the physics of mean--field
dynamo and the physical mechanisms behind intermittent behavior observed in
stellar magnetic field variability are discussed.Comment: 19 pages, 13 figure
Existence, uniqueness and analyticity of space-periodic solutions to the regularised long-wave equation
We consider space-periodic evolutionary and travelling-wave solutions to the
regularised long-wave equation (RLWE) with damping and forcing. We establish
existence, uniqueness and smoothness of the evolutionary solutions for smooth
initial conditions, and global in time spatial analyticity of such solutions
for analytical initial conditions. The width of the analyticity strip decays at
most polynomially. We prove existence of travelling-wave solutions and
uniqueness of travelling waves of a sufficiently small norm. The importance of
damping is demonstrated by showing that the problem of finding travelling-wave
solutions to the undamped RLWE is not well-posed. Finally, we demonstrate the
asymptotic convergence of the power series expansion of travelling waves for a
weak forcing.Comment: 29 pp., 4 figures, 44 reference
The divergence of private from social costs in rural-urban migration: a case study of Nairobi, Kenya
In Developing Economies the level of urban wages tends
to induce more people to seek employment in the towns than can be
employed at this wage level. The existence of these urban unemployed
causes the private costs of migration to diverge from the social
costs. The individual rural resident decides to remain or migrate
on the basis of perceived private costs of migration. The effect
of a decision to migrate on the economy is the social cost of
migration. In our study we consider the determinants of different
levels of private and social costs associated with different stocks
of urban unemployed. In addition, utilizing survey data on Nairobi,
Kenya, an attempt is made to quantify, the major private and social
costs of migration to determine whether they diverge significantly.
On the basis of these estimates some policy options for limiting
urban unemployment caused by urban in-migration are considered
Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma
Alfv\'en waves may be generated via mode conversion from fast
magneto-acoustic waves near their reflection level in the solar atmosphere,
with implications both for coronal oscillations and for active region
helio-seismology. In active regions this reflection typically occurs high
enough that the Alfv\'en speed greatly exceeds the sound speed , well
above the level where the fast and slow modes interact. In order to focus
on the fundamental characteristics of fast/Alfv\'en conversion, stripped of
unnecessary detail, it is therefore useful to freeze out the slow mode by
adopting the gravitationally stratified cold MHD model . This provides a
benchmark for fast-to-Alfv\'en mode conversion in more complex atmospheres.
Assuming a uniform inclined magnetic field and an exponential Alfv\'en speed
profile with density scale height , the Alfv\'en conversion coefficient
depends on three variables only; the dimensionless
transverse-to-the-stratification wavenumber , the magnetic field
inclination from the stratification direction , and the polarization
angle of the wavevector relative to the plane containing the
stratification and magnetic field directions. We present an extensive
exploration of mode conversion in this parameter space and conclude that
near-total conversion to outward-propagating Alfv\'en waves typically occurs
for small and large (--), though it is
absent entirely when is exactly zero (vertical field). For wavenumbers
of helioseismic interest, the conversion region is broad enough to encompass
the whole chromosphere.Comment: 14 pages plus supplementary tables. Astrophys J (accepted 25 May
2011). Two ancillary animations (animated gif) attache
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