111,350 research outputs found
Quasilinear Drift Of Cosmic Rays In Weak Turbulent Electromagnetic Fields
A general quasilinear transport parameter for particle drift in arbitrary
turbulence geometry is presented. The new drift coefficient is solely
characterized by a nonresonant term and is evaluated for slab and
two-dimensional turbulence geometry. The calculations presented here
demonstrate that fluctuating electric fields are a key quantity for
understanding quasilinear particle drift in slab geometry. It is shown that
particle drift does not exist in unpolarized and purely magnetic slab
fluctuations. This is in stark contrast to previous models, which are
restricted to slab geometry and the field line random walk limit. The
evaluation of the general transport parameter for two-dimensional turbulence
geometry, presented here for the first time for dynamical magnetic turbulence,
results in a drift coefficient valid for a magnetic power spectrum and
turbulence decay rate varying arbitrarily in wavenumber. For a two-component,
slab/two-dimensional turbulence model, numerical calculations are presented.
The new quasilinear drift, induced by the magnetic perturbations, is compared
with a standard drift expression related to the curvature and gradient of an
unperturbed heliospheric background magnetic field. The considerations
presented here offer a solid ground and natural explanation for the hitherto
puzzling observation that drift models often describe observations much better
when drift effects are reduced.Comment: 23 pages, 6 figures, accepted for publication in Ap
Critical behavior and entanglement of the random transverse-field Ising model between one and two dimensions
We consider disordered ladders of the transverse-field Ising model and study
their critical properties and entanglement entropy for varying width, , by numerical application of the strong disorder renormalization group
method. We demonstrate that the critical properties of the ladders for any
finite are controlled by the infinite disorder fixed point of the random
chain and the correction to scaling exponents contain information about the
two-dimensional model. We calculate sample dependent pseudo-critical points and
study the shift of the mean values as well as scaling of the width of the
distributions and show that both are characterized by the same exponent,
. We also study scaling of the critical magnetization, investigate
critical dynamical scaling as well as the behavior of the critical entanglement
entropy. Analyzing the -dependence of the results we have obtained accurate
estimates for the critical exponents of the two-dimensional model:
, and .Comment: 10 pages, 9 figure
The effect of realistic geometries on the susceptibility-weighted MR signal in white matter
Purpose: To investigate the effect of realistic microstructural geometry on
the susceptibility-weighted magnetic resonance (MR) signal in white matter
(WM), with application to demyelination.
Methods: Previous work has modeled susceptibility-weighted signals under the
assumption that axons are cylindrical. In this work, we explore the
implications of this assumption by considering the effect of more realistic
geometries. A three-compartment WM model incorporating relevant properties
based on literature was used to predict the MR signal. Myelinated axons were
modeled with several cross-sectional geometries of increasing realism: nested
circles, warped/elliptical circles and measured axonal geometries from electron
micrographs. Signal simulations from the different microstructural geometries
were compared to measured signals from a Cuprizone mouse model with varying
degrees of demyelination.
Results: Results from simulation suggest that axonal geometry affects the MR
signal. Predictions with realistic models were significantly different compared
to circular models under the same microstructural tissue properties, for
simulations with and without diffusion.
Conclusion: The geometry of axons affects the MR signal significantly.
Literature estimates of myelin susceptibility, which are based on fitting
biophysical models to the MR signal, are likely to be biased by the assumed
geometry, as will any derived microstructural properties.Comment: Accepted March 4 2017, in publication at Magnetic Resonance in
Medicin
Correlations between reflected and transmitted intensity patterns emerging from opaque disordered media
The propagation of monochromatic light through a scattering medium produces
speckle patterns in reflection and transmission, and the apparent randomness of
these patterns prevents direct imaging through thick turbid media. Yet, since
elastic multiple scattering is fundamentally a linear and deterministic
process, information is not lost but distributed among many degrees of freedom
that can be resolved and manipulated. Here we demonstrate experimentally that
the reflected and transmitted speckle patterns are correlated, even for opaque
media with thickness much larger than the transport mean free path, proving
that information survives the multiple scattering process and can be recovered.
The existence of mutual information between the two sides of a scattering
medium opens up new possibilities for the control of transmitted light without
any feedback from the target side, but using only information gathered from the
reflected speckle.Comment: 6 pages, 4 figure
Geometric phases in electric dipole searches with trapped spin-1/2 particles in general fields and measurement cells of arbitrary shape with smooth or rough walls
The important role of geometric phases in searches for a permanent electric
dipole moment of the neutron, using Ramsey separated oscillatory field nuclear
magnetic resonance, was first noted by Commins and investigated in detail by
Pendlebury et al. Their analysis was based on the Bloch equations. In
subsequent work using the spin density matrix Lamoreaux and Golub showed the
relation between the frequency shifts and the correlation functions of the
fields seen by trapped particles in general fields (Redfield theory). More
recently we presented a solution of the Schr\"odinger equation for spin-
particles in circular cylindrical traps with smooth walls and exposed to
arbitrary fields [Steyerl et al.] Here we extend this work to show how the
Redfield theory follows directly from the Schr\"odinger equation solution. This
serves to highlight the conditions of validity of the Redfield theory, a
subject of considerable discussion in the literature [e.g., Nicholas et al.]
Our results can be applied where the Redfield result no longer holds, such as
observation times on the order of or shorter than the correlation time and
non-stochastic systems and thus we can illustrate the transient spin dynamics,
i.e. the gradual development of the shift with increasing time subsequent to
the start of the free precession. We consider systems with rough, diffuse
reflecting walls, cylindrical trap geometry with arbitrary cross section, and
field perturbations that do not, in the frame of the moving particles, average
to zero in time. We show by direct, detailed, calculation the agreement of the
results from the Schr\"odinger equation with the Redfield theory for the cases
of a rectangular cell with specular walls and of a circular cell with diffuse
reflecting walls.Comment: 20 pages, 8 figure
Theoretical study of optical fiber Raman polarizers with counterpropagating beams
The theory of two counter-propagating polarized beams interacting in a
randomly birefringent fiber via the Kerr and Raman effects is developed and
applied to the quantitative description of Raman polarizers in the undepleted
regime. Here Raman polarizers, first reported by Martinelli et. al. [Opt.
Express. 17, 947 (2009)], are understood as Raman amplifiers operating in the
regime in which an initially weak unpolarized beam is converted into an
amplified fully polarized beam towards the fiber output. Three parameters are
selected for the characterization of a Raman polarizer: the degree of
polarization of the outcoming beam, its state of polarization, and its gain.
All of these parameters represent quantities that are averaged over all random
polarization states of the initially unpolarized signal beam. The presented
theory is computer friendly and applicable to virtually all practically
relevant situations, including the case of co-propagating beams, and in
particular to the undepleted as well as the depleted regimes of the Raman
polarizer.Comment: 16 pages, 5 figures, to be submitted to the Journal of Lightwave
Technolog
Robust Detection of Moving Human Target in Foliage-Penetration Environment Based on Hough Transform
Attention has been focused on the robust moving human target detection in foliage-penetration environment, which presents a formidable task in a radar system because foliage is a rich scattering environment with complex multipath propagation and time-varying clutter. Generally, multiple-bounce returns and clutter are additionally superposed to direct-scatter echoes. They obscure true target echo and lead to poor visual quality time-range image, making target detection particular difficult. Consequently, an innovative approach is proposed to suppress clutter and mitigate multipath effects. In particular, a clutter suppression technique based on range alignment is firstly applied to suppress the time-varying clutter and the instable antenna coupling. Then entropy weighted coherent integration (EWCI) algorithm is adopted to mitigate the multipath effects. In consequence, the proposed method effectively reduces the clutter and ghosting artifacts considerably. Based on the high visual quality image, the target trajectory is detected robustly and the radial velocity is estimated accurately with the Hough transform (HT). Real data used in the experimental results are provided to verify the proposed method
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