91,060 research outputs found
Power law spectra and intermittent fluctuations due to uncorrelated Lorentzian pulses
A stochastic model for intermittent fluctuations due to a super-position of
uncorrelated Lorentzian pulses is presented. For constant pulse duration, this
is shown to result in an exponential power spectral density for the stationary
process. A random distribution of pulse durations modifies the frequency
spectrum and several examples are shown to result in power law spectra. The
distribution of pulse durations does not influence the characteristic function
and thus neither the moments nor the probability density function for the
random variable. It is demonstrated that the fluctuations are intrinsically
intermittent through a large excess kurtosis moment in the limit of weak pulse
overlap. These results allow to estimate the basic properties of fluctuations
from measurement data and describe the diversity of frequency spectra reported
from measurements in magnetized plasmas.Comment: 12 pages, 4 figure
Intermittent fluctuations due to uncorrelated Lorentzian pulses
Fluctuations due to a super-position of uncorrelated Lorentzian pulses with a
random distribution of amplitudes and duration times are considered. These are
demonstrated to be strongly intermittent in the limit of weak pulse overlap,
resulting in large skewness and flatness moments. The characteristic function
and the lowest order moments are derived, revealing a parabolic relationship
between the skewness and flatness moments. Numerical integration reveals the
probability density functions in the case of exponential and Laplace
distributed pulse amplitudes. This stochastic model describes the intermittent
fluctuations and probability densities with exponential tails commonly observed
in turbulent fluids and magnetized plasmas.Comment: 12 pages, 3 figure
Preface "Nonlinear processes in oceanic and atmospheric flows"
Nonlinear phenomena are essential ingredients in many oceanic and atmospheric
processes, and successful understanding of them benefits from multidisciplinary
collaboration between oceanographers, meteorologists, physicists and
mathematicians. The present Special Issue on ``Nonlinear Processes in Oceanic
and Atmospheric Flows'' contains selected contributions from attendants to the
workshop which, in the above spirit, was held in Castro Urdiales, Spain, in
July 2008. Here we summarize the Special Issue contributions, which include
papers on the characterization of ocean transport in the Lagrangian and in the
Eulerian frameworks, generation and variability of jets and waves, interactions
of fluid flow with plankton dynamics or heavy drops, scaling in meteorological
fields, and statistical properties of El Ni\~no Southern Oscillation.Comment: This is the introductory article to a Special Issue on "Nonlinear
Processes in Oceanic and Atmospheric Flows'', published in the journal
Nonlinear Processes in Geophysics, where the different contributions are
summarized. The Special Issue itself is freely available from
http://www.nonlin-processes-geophys.net/special_issue103.htm
Explicit towers of Drinfeld modular curves
We give explicit equations for the simplest towers of Drinfeld modular curves
over any finite field, and observe that they coincide with the asymptotically
optimal towers of curves constructed by Garcia and Stichtenoth.Comment: 10 pages. For mini-symposium on "curves over finite fields and codes"
at the 3rd European Congress in Barcelona 7/2000 Revised to correct minor
typographical and grammatical error
Kalikow-type decomposition for multicolor infinite range particle systems
We consider a particle system on with real state space and
interactions of infinite range. Assuming that the rate of change is continuous
we obtain a Kalikow-type decomposition of the infinite range change rates as a
mixture of finite range change rates. Furthermore, if a high noise condition
holds, as an application of this decomposition, we design a feasible perfect
simulation algorithm to sample from the stationary process. Finally, the
perfect simulation scheme allows us to forge an algorithm to obtain an explicit
construction of a coupling attaining Ornstein's -distance for two
ordered Ising probability measures.Comment: Published in at http://dx.doi.org/10.1214/12-AAP882 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
White dwarf cooling sequences and cosmochronology
The evolution of white dwarfs is a simple gravothermal process. This means
that their luminosity function, i.e. the number of white dwarfs per unit
bolometric magnitude and unit volume as a function of bolometric magnitude, is
a monotonically increasing function that decreases abruptly as a consequence of
the finite age of the Galaxy. The precision and the accuracy of the white dwarf
luminosity functions obtained with the recent large surveys together with the
improved quality of the theoretical models of evolution of white dwarfs allow
to feed the hope that in a near future it will be possible to reconstruct the
history of the different Galactic populations.Comment: Proceedings of the 40th Liege International Astrophysical Colloquium:
Aging low mass stars: from red giants to white dwarf
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