2,438 research outputs found
Positive Feedback, Memory and the Predictability of Earthquakes
We review the "critical point" concept for large earthquakes and enlarge it
in the framework of so-called "finite-time singularities". The singular
behavior associated with accelerated seismic release is shown to result from a
positive feedback of the seismic activity on its release rate. The most
important mechanisms for such positive feedback are presented. We introduce and
solve analytically a novel simple model of geometrical positive feedback in
which the stress shadow cast by the last large earthquake is progressively
fragmented by the increasing tectonic stress. Finally, we present a somewhat
speculative figure that tends to support a mechanism based on the decay of
stress shadows. This figure suggests that a large earthquake in Southern
California of size similar to the 1812 great event is maturing.Comment: PostScript document of 18 pages + 2 eps figure
The effect of heavy element opacity on pre-main sequence Li depletion
Recent 3-D analysis of the solar spectrum data suggests a significant change
of the solar chemical composition. This may affect the temporal evolution of
the surface abundance of light elements since the extension of the convective
envelope is largely affected by the internal opacity value. We analyse the
influence of the adopted solar mixture on the opacity in the convective
envelope of pre-main sequence (PMS) stars and thus on PMS lithium depletion.
The surface Li abundance depends on the relative efficiency of several
processes, some of them still not known with the required precision; this paper
thus analyses one of the aspects of this ``puzzle''. Focusing on PMS evolution,
where the largest amount of Li burning occurs, we computed stellar models for
three selected masses (0.8, 1.0 and 1.2 Msun, with Z=0.013, Y=0.27, alpha=1.9)
by varying the chemical mixture, that is the internal element distribution in
Z. We analysed the contribution of the single elements to the opacity at the
temperatures and densities of interest for Li depletion. Several mixtures were
obtained by varying the abundance of the most important elements one at a time;
we then calculated the corresponding PMS Li abundance evolution. We found that
a mixture variation does change the Li abundance: at fixed total metallicity,
the Li depletion increases when increasing the fraction of elements heavier
than O.Comment: A&A accepted, 11 pages, 18 eps figure
Energy and Reconstruction for Cosmic-Ray Events Recorded by a Prototype Station of the IceCube Surface Enhancement
The IceTop array, located at the surface of the IceCube Neutrino Observatory at the South Pole, is currently used as a veto for the in-ice neutrino detection as well as a cosmic-ray detector. Over the years, additional snow has accumulated on the IceTop detectors leading to a reduction in sensitivity and resolution.
In order to mitigate this issue as well as further increase the accuracy of cosmic-ray measurements, a detector enhancement is planned in the next few years. The enhanced array will consist of 32 stations, each comprising 8 scintillation detectors and 3 radio antennas, and will span an area of approximately 1km. Specifically, upgrading IceTop with radio antennas will provide precise measurements, a variable widely used to reconstruct the composition of cosmic rays.
In January 2020, a complete prototype station
was deployed at the South Pole. Following the measurement of cosmic-ray events with the antennas,
we developed the tools necessary to use a template-matching method for energy and reconstruction.
This template method uses Monte-Carlo simulations and compares them to recorded data. Thus, a set of simulated air showers is created using air shower parameters reconstructed by IceTop as input to the CORSIKA/CoREAS simulation software for each of the measured events.
In this work, the method is applied to measured events, and we will show that it works for a third of the events in the sample. Subsequently, the technique is modified into a log-likelihood minimization, and the results obtained with a simulations-only study of different parameters are discussed. This work concludes with a confirmation that the template method works for air showers recorded with a frequency band of 80 to 300MHz,
and shows that the log-likelihood method can potentially increase the accuracy of reconstruction for the complete planned array
Using the seismology of non-magnetic chemically peculiar stars as a probe of dynamical processes in stellar interiors
Chemical composition is a good tracer of hydrodynamical processes that occur
in stars as they often lead to mixing and particle transport. By comparing
abundances predicted by models and those observed in stars we can infer some
constraints on those mixing processes. As pulsations in stars are often very
sensitive to chemical composition, we can use asteroseismology to probe the
internal chemical composition of stars where no direct observations are
possible. In this paper I focus on main sequence stars Am, lambda bootis, and
HgMn stars and discuss what we can learn of mixing processes in those stars
from seismology.Comment: 10 pages,6 figures. accepted in Journal of astrophysics and
astronomy. proceedings of aries conferemce on asteroseismology. december 200
Zipf's law in Multifragmentation
We discuss the meaning of Zipf's law in nuclear multifragmentation. We remark
that Zipf's law is a consequence of a power law fragment size distribution with
exponent . We also recall why the presence of such distribution
is not a reliable signal of a liquid-gas phase transition
Scaling Analysis and Evolution Equation of the North Atlantic Oscillation Index Fluctuations
The North Atlantic Oscillation (NAO) monthly index is studied from 1825 till
2002 in order to identify the scaling ranges of its fluctuations upon different
delay times and to find out whether or not it can be regarded as a Markov
process. A Hurst rescaled range analysis and a detrended fluctuation analysis
both indicate the existence of weakly persistent long range time correlations
for the whole scaling range and time span hereby studied. Such correlations are
similar to Brownian fluctuations. The Fokker-Planck equation is derived and
Kramers-Moyal coefficients estimated from the data. They are interpreted in
terms of a drift and a diffusion coefficient as in fluid mechanics. All partial
distribution functions of the NAO monthly index fluctuations have a form close
to a Gaussian, for all time lags, in agreement with the findings of the scaling
analyses. This indicates the lack of predictive power of the present NAO
monthly index. Yet there are some deviations for large (and thus rare) events.
Whence suggestions for other measurements are made if some improved
predictability of the weather/climate in the North Atlantic is of interest. The
subsequent Langevin equation of the NAO signal fluctuations is explicitly
written in terms of the diffusion and drift parameters, and a characteristic
time scale for these is given in appendix.Comment: 6 figures, 54 refs., 16 pages; submitted to Int. J. Mod. Phys. C:
Comput. Phy
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