12,482 research outputs found
Noise-Induced Linearisation and Delinearisation
It is demonstrated, by means of analogue electronic simulation and
theoretically, that external noise can markedly change the character of the
response of a nonlinear system to a low-frequency periodic field. In general,
noise of sufficient intensity {\it linearises} the response. For certain
parameter ranges in particular cases, however, an increase in the noise
intensity can sometime have the opposite effect and is shown to {\it
delinearise} the response. The physical origins of these contrary behaviours
are discussed.Comment: 17 pages. No special macros. Figures on reques
Wavelet Deconvolution in a Periodic Setting with Long-Range Dependent Errors
In this paper, a hard thresholding wavelet estimator is constructed for a
deconvolution model in a periodic setting that has long-range dependent noise.
The estimation paradigm is based on a maxiset method that attains a near
optimal rate of convergence for a variety of L_p loss functions and a wide
variety of Besov spaces in the presence of strong dependence. The effect of
long-range dependence is detrimental to the rate of convergence. The method is
implemented using a modification of the WaveD-package in R and an extensive
numerical study is conducted. The numerical study supplements the theoretical
results and compares the LRD estimator with na\"ively using the standard WaveD
approach
Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence
We employ simulations of supersonic super-Alfvenic turbulence decay as a
benchmark test problem to assess and compare the performance of nine
astrophysical MHD methods actively used to model star formation. The set of
nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER,
and ZEUS. We present a comprehensive set of statistical measures designed to
quantify the effects of numerical dissipation in these MHD solvers. We compare
power spectra for basic fields to determine the effective spectral bandwidth of
the methods and rank them based on their relative effective Reynolds numbers.
We also compare numerical dissipation for solenoidal and dilatational velocity
components to check for possible impacts of the numerics on small-scale density
statistics. Finally, we discuss convergence of various characteristics for the
turbulence decay test and impacts of various components of numerical schemes on
the accuracy of solutions. We show that the best performing codes employ a
consistently high order of accuracy for spatial reconstruction of the evolved
fields, transverse gradient interpolation, conservation law update step, and
Lorentz force computation. The best results are achieved with divergence-free
evolution of the magnetic field using the constrained transport method, and
using little to no explicit artificial viscosity. Codes which fall short in one
or more of these areas are still useful, but they must compensate higher
numerical dissipation with higher numerical resolution. This paper is the
largest, most comprehensive MHD code comparison on an application-like test
problem to date. We hope this work will help developers improve their numerical
algorithms while helping users to make informed choices in picking optimal
applications for their specific astrophysical problems.Comment: 17 pages, 5 color figures, revised version to appear in ApJ, 735,
July 201
Fearful faces have a sensory advantage in the competition for awareness
Only a subset of visual signals give rise to a conscious percept. Threat signals, such as fearful faces, are particularly salient to human vision. Research suggests that fearful faces are evaluated without awareness and preferentially promoted to conscious perception. This agrees with evolutionary theories that posit a dedicated pathway specialized in processing threat-relevant signals. We propose an alternative explanation for this "fear advantage." Using psychophysical data from continuous flash suppression (CFS) and masking experiments, we demonstrate that awareness of facial expressions is predicted by effective contrast: the relationship between their Fourier spectrum and the contrast sensitivity function. Fearful faces have higher effective contrast than neutral expressions and this, not threat content, predicts their enhanced access to awareness. Importantly, our findings do not support the existence of a specialized mechanism that promotes threatening stimuli to awareness. Rather, our data suggest that evolutionary or learned adaptations have molded the fearful expression to exploit our general-purpose sensory mechanisms
Dynamics and hysteresis in square lattice artificial spin-ice
Dynamical effects under geometrical frustration are considered in a model for
artificial spin ice on a square lattice in two dimensions. Each island of the
spin ice has a three-component Heisenberg-like dipole moment subject to shape
anisotropies that influence its direction. The model has real dynamics,
including rotation of the magnetic degrees of freedom, going beyond the
Ising-type models of spin ice. The dynamics is studied using a Langevin
equation solved via a second order Heun algorithm. Thermodynamic properties
such as the specific heat are presented for different couplings. A peak in
specific heat is related to a type of melting-like phase transition present in
the model. Hysteresis in an applied magnetic field is calculated for model
parameters where the system is able to reach thermodynamic equilibrium.Comment: Revised versio
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