220 research outputs found
Quantifying and containing the curse of high resolution coronal imaging
Future missions such as Solar Orbiter (SO), InterHelioprobe, or Solar Probe
aim at approaching the Sun closer than ever before, with on board some high
resolution imagers (HRI) having a subsecond cadence and a pixel area of about
at the Sun during perihelion. In order to guarantee their scientific
success, it is necessary to evaluate if the photon counts available at these
resolution and cadence will provide a sufficient signal-to-noise ratio (SNR).
We perform a first step in this direction by analyzing and characterizing the
spatial intermittency of Quiet Sun images thanks to a multifractal analysis.
We identify the parameters that specify the scale-invariance behavior. This
identification allows next to select a family of multifractal processes, namely
the Compound Poisson Cascades, that can synthesize artificial images having
some of the scale-invariance properties observed on the recorded images.
The prevalence of self-similarity in Quiet Sun coronal images makes it
relevant to study the ratio between the SNR present at SoHO/EIT images and in
coarsened images. SoHO/EIT images thus play the role of 'high resolution'
images, whereas the 'low-resolution' coarsened images are rebinned so as to
simulate a smaller angular resolution and/or a larger distance to the Sun. For
a fixed difference in angular resolution and in Spacecraft-Sun distance, we
determine the proportion of pixels having a SNR preserved at high resolution
given a particular increase in effective area. If scale-invariance continues to
prevail at smaller scales, the conclusion reached with SoHO/EIT images can be
transposed to the situation where the resolution is increased from SoHO/EIT to
SO/HRI resolution at perihelion.Comment: 25 pages, 1 table, 7 figure
Size matters: some stylized facts of the stock market revisited
We reanalyze high resolution data from the New York Stock Exchange and find a
monotonic (but not power law) variation of the mean value per trade, the mean
number of trades per minute and the mean trading activity with company
capitalization. We show that the second moment of the traded value distribution
is finite. Consequently, the Hurst exponents for the corresponding time series
can be calculated. These are, however, non-universal: The persistence grows
with larger capitalization and this results in a logarithmically increasing
Hurst exponent. A similar trend is displayed by intertrade time intervals.
Finally, we demonstrate that the distribution of the intertrade times is better
described by a multiscaling ansatz than by simple gap scaling.Comment: 10 pages, 13 figures, 2 tables, accepted to Eur. Phys. J. B, updated
references, fixed some minor error
Scaling and multiscaling in financial series: a simple model
We propose a simple stochastic volatility model which is analytically
tractable, very easy to simulate and which captures some relevant stylized
facts of financial assets, including scaling properties. In particular, the
model displays a crossover in the log-return distribution from power-law tails
(small time) to a Gaussian behavior (large time), slow decay in the volatility
autocorrelation and multiscaling of moments. Despite its few parameters, the
model is able to fit several key features of the time series of financial
indexes, such as the Dow Jones Industrial Average, with a remarkable accuracy.Comment: 32 pages, 5 figures. Substantial revision, following the referee's
suggestions. Version to appear in Adv. in Appl. Proba
Multifractality in the stock market: price increments versus waiting times
By applying the multifractal detrended fluctuation analysis to the
high-frequency tick-by-tick data from Deutsche B\"orse both in the price and in
the time domains, we investigate multifractal properties of the time series of
logarithmic price increments and inter-trade intervals of time. We show that
both quantities reveal multiscaling and that this result holds across different
stocks. The origin of the multifractal character of the corresponding dynamics
is, among others, the long-range correlations in price increments and in
inter-trade time intervals as well as the non-Gaussian distributions of the
fluctuations. Since the transaction-to-transaction price increments do not
strongly depend on or are almost independent of the inter-trade waiting times,
both can be sources of the observed multifractal behaviour of the fixed-delay
returns and volatility. The results presented also allow one to evaluate the
applicability of the Multifractal Model of Asset Returns in the case of
tick-by-tick data.Comment: Physica A, in prin
Universality issues in surface kinetic roughening of thin solid films
Since publication of the main contributions on the theory of kinetic
roughening more than fifteen years ago, many works have been reported on
surface growth or erosion that employ the framework of dynamic scaling. This
interest was mainly due to the predicted existence of just a few universality
classes to describe the statistical properties of the morphology of growing
surfaces and interfaces that appear in a wide range of physical systems.
Nowadays, this prediction seems to be inaccurate. This situation has caused a
clear detriment of these studies in spite of the undeniable existence of
kinetic roughening in many different real systems, and without a clear
understanding of the reasons behind the mismatch between theoretical
expectations and experimental observations. In this chapter we aim to explore
existing problems and shortcomings of both the theoretical and experimental
approaches, focusing mainly on growth of thin solid films. Our analysis
suggests that the theoretical framework as yet is not complete, while more
systematic and consistent experiments need to be performed. Once these issues
are taken into account, a more consistent and useful theory of kinetic
roughening might develop.Comment: Review article to appear in ``Advances in Condensed Matter and
Statistical Mechanics", ed. E. Korutcheva and R. Cuerno. To be published by
Nova Science Publishers. 22 pages. 4 eps figure
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