45,047 research outputs found
A three dimensional model of the Venusian thermosphere with superrotation
An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory
Enhancement of Kerr nonlinearity via multi-photon coherence
We propose a new method of resonant enhancement of optical Kerr nonlinearity
using multi-level atomic coherence. The enhancement is accompanied by
suppression of the other linear and nonlinear susceptibility terms of the
medium. We show that the effect results in a modification of the nonlinear
Faraday rotation of light propagating in an Rb87 vapor cell by changing the
ellipticity of the light.Comment: 4 pages, 3 figures Submitted to Optics Letter
On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members
The metallicity distribution function (MDF) for the old red-giant stars in
the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually
identical with the MDF for the old-disk stars in the LMC and also strongly
resembles the halo MDF in M31. These galaxies all have high mean halo
metallicities ( ~ -0.4$) with very small proportions of low-metallicity
stars. These observations reinforce the view that metal-rich halos are quite
normal for large galaxies of all types. Such systems are unlikely to have built
up by accretion of pre-existing, gas-free small satellite galaxies, unless
these satellites had an extremely shallow mass distribution (d log N / d log M
> -1). We suggest that the halo of NGC 5128 is more likely to have assembled
from hierarchical merging of gas-rich lumps in which the bulk of star formation
took place during or after the merger stage.Comment: 10 pages, LaTeX, plus 3 figures in separate postscript files;
Astronomical Journal, in press for December 200
Zero range model of traffic flow
A multi--cluster model of traffic flow is studied, in which the motion of
cars is described by a stochastic master equation. Assuming that the escape
rate from a cluster depends only on the cluster size, the dynamics of the model
is directly mapped to the mathematically well-studied zero-range process.
Knowledge of the asymptotic behaviour of the transition rates for large
clusters allows us to apply an established criterion for phase separation in
one-dimensional driven systems. The distribution over cluster sizes in our
zero-range model is given by a one--step master equation in one dimension. It
provides an approximate mean--field dynamics, which, however, leads to the
exact stationary state. Based on this equation, we have calculated the critical
density at which phase separation takes place. We have shown that within a
certain range of densities above the critical value a metastable homogeneous
state exists before coarsening sets in. Within this approach we have estimated
the critical cluster size and the mean nucleation time for a condensate in a
large system. The metastablity in the zero-range process is reflected in a
metastable branch of the fundamental flux--density diagram of traffic flow. Our
work thus provides a possible analytical description of traffic jam formation
as well as important insight into condensation in the zero-range process.Comment: 10 pages, 13 figures, small changes are made according to finally
accepted version for publication in Phys. Rev.
Fluctuations of the heat flux of a one-dimensional hard particle gas
Momentum-conserving one-dimensional models are known to exhibit anomalous
Fourier's law, with a thermal conductivity varying as a power law of the system
size. Here we measure, by numerical simulations, several cumulants of the heat
flux of a one-dimensional hard particle gas. We find that the cumulants, like
the conductivity, vary as power laws of the system size. Our results also
indicate that cumulants higher than the second follow different power laws when
one compares the ring geometry at equilibrium and the linear case in contact
with two heat baths (at equal or unequal temperatures). keywords: current
fluctuations, anomalous Fourier law, hard particle gasComment: 5 figure
Properties of the mechanosensitive channel MscS pore revealed by tryptophan scanning mutagenesis
Funding This work was supported by a Wellcome Trust Programme grant [092552/A/10/Z awarded to I.R.B., S.M., J. H. Naismith (University of St Andrews, St Andrews, U.K.), and S. J. Conway (University of Oxford, Oxford, U.K.)] (T.R. and M.D.E.), by a BBSRC grant (A.R.) [BB/H017917/1 awarded to I.R.B., J. H. Naismith, and O. Schiemann (University of St Andrews)], by a Leverhulme Emeritus Fellowship (EM-2012-060\2), and by a CEMI grant to I.R.B. from the California Institute of Technology. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013 FP7/2007-2011) under Grant PITN-GA-2011-289384 (FP7-PEOPLE-2011-ITN NICHE) (H.G.) (awarded to S.M.).Peer reviewedPublisher PD
On a random walk with memory and its relation to Markovian processes
We study a one-dimensional random walk with memory in which the step lengths
to the left and to the right evolve at each step in order to reduce the
wandering of the walker. The feedback is quite efficient and lead to a
non-diffusive walk. The time evolution of the displacement is given by an
equivalent Markovian dynamical process. The probability density for the
position of the walker is the same at any time as for a random walk with
shrinking steps, although the two-time correlation functions are quite
different.Comment: 10 pages, 4 figure
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