138 research outputs found
A stochastic theory for temporal fluctuations in self-organized critical systems
A stochastic theory for the toppling activity in sandpile models is
developed, based on a simple mean-field assumption about the toppling process.
The theory describes the process as an anti-persistent Gaussian walk, where the
diffusion coefficient is proportional to the activity. It is formulated as a
generalization of the It\^{o} stochastic differential equation with an
anti-persistent fractional Gaussian noise source. An essential element of the
theory is re-scaling to obtain a proper thermodynamic limit, and it captures
all temporal features of the toppling process obtained by numerical simulation
of the Bak-Tang-Wiesenfeld sandpile in this limit.Comment: 9 pages, 4 figure
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
Is there long-range memory in solar activity on time scales shorter than the sunspot period?
The sunspot number (SSN), the total solar irradiance (TSI), a TSI
reconstruction, and the solar flare index (SFI), are analyzed for long-range
persistence (LRP). Standard Hurst analysis yields , which
suggests strong LRP. However, solar activity time series are non-stationary due
to the almost periodic 11 year smooth component, and the analysis does not give
the correct for the stochastic component. Better estimates are obtained by
detrended fluctuations analysis (DFA), but estimates are biased and errors are
large due to the short time records. These time series can be modeled as a
stochastic process of the form , where
is the smooth component, and is a stationary fractional noise
with Hurst exponent . From ensembles of numerical solutions to the
stochastic model, and application of Bayes' theorem, we can obtain bias and
error bars on and also a test of the hypothesis that a process is
uncorrelated (). The conclusions from the present data sets are that
SSN, TSI and TSI reconstruction almost certainly are long-range persistent, but
with most probable value . The SFI process, however, is either
very weakly persistent () or completely uncorrelated. Some differences
between stochastic properties of the TSI and its reconstruction indicate some
error in the reconstruction scheme.Comment: 23 pages, 12 figure
Multifractal modeling of short-term interest rates
We propose a multifractal model for short-term interest rates. The model is a
version of the Markov-Switching Multifractal (MSM), which incorporates the
well-known level effect observed in interest rates. Unlike previously suggested
models, the level-MSM model captures the power-law scaling of the structure
functions and the slowly decaying dependency in the absolute value of returns.
We apply the model to the Norwegian Interbank Offered Rate with three months
maturity (NIBORM3) and the U.S. Treasury Bill with three months maturity
(TBM3). The performance of the model is compared to level-GARCH models,
level-EGARCH models and jump-diffusions. For the TBM3 data the multifractal
out-performs all the alternatives considered.Comment: 16 pages, 3 figures, 7 table
Scale-free vortex cascade emerging from random forcing in a strongly coupled system
The notions of self-organised criticality (SOC) and turbulence are
traditionally considered to be applicable to disjoint classes of phenomena.
Nevertheless, scale-free burst statistics is a feature shared by turbulent as
well as self-organised critical dynamics. It has also been suggested that
another shared feature is universal non-gaussian probability density functions
(PDFs) of global fluctuations. Here, we elucidate the unifying aspects through
analysis of data from a laboratory dusty plasma monolayer. We compare analysis
of experimental data with simulations of a two-dimensional (2D) many-body
system, of 2D fluid turbulence, and a 2D SOC model, all subject to random
forcing at small scales. The scale-free vortex cascade is apparent from
structure functions as well as spatio-temporal avalanche analysis, the latter
giving similar results for the experimental and all model systems studied. The
experiment exhibits global fluctuation statistics consistent with a
non-gaussian universal PDF, but the model systems yield this result only in a
restricted range of forcing conditions
Approximated maximum likelihood estimation in multifractal random walks
We present an approximated maximum likelihood method for the multifractal
random walk processes of [E. Bacry et al., Phys. Rev. E 64, 026103 (2001)]. The
likelihood is computed using a Laplace approximation and a truncation in the
dependency structure for the latent volatility. The procedure is implemented as
a package in the R computer language. Its performance is tested on synthetic
data and compared to an inference approach based on the generalized method of
moments. The method is applied to estimate parameters for various financial
stock indices.Comment: 8 pages, 3 figures, 2 table
Variation in carbon footprint of milk due to management differences between Swedish dairy farms
To identify mitigation options to reduce greenhouse gas (GHG) emissions from milk production (i.e. the carbon footprint (CF) of milk), this study examined the variation in GHG emissions among dairy farms using data from previous CF studies on Swedish milk. Variation between farms in these production data, which were found to have a strong influence on milk CF were obtained from existing databases of e.g. 1051 dairy farms in Sweden in 2005. Monte Carlo analysis was used to analyse the impact of variations in seven important parameters on milk CF concerning milk yield (energy corrected milk (ECM) produced and delivered), feed dry matter intake (DMI), enteric methane emissions, N content in feed DMI, N-fertiliser rate and diesel used on farm. The largest between farm variation among the analysed production data were N-fertiliser rate (kg/ha) and diesel used (l/ha) on farm (coefficient of variation (CV) 31-38%). For the parameters concerning milk yield and feed DMI the CV was approx. 11 and 8%, respectively. The smallest variation in production data was found for N content in feed DMI. According to the Monte Carlo analysis, these variations in production data led to a variation in milk CF of between 0.94 and 1.33 kg CO2 equivalents (CO2e) per kg ECM, with an average value of 1.13 kg/CO2e kg ECM. We consider that this variation of ±17% that was found based on the used farm data would be even greater if all Swedish dairy farms were included, as the sample of farms in this study was not totally unbiased. The variation identified in milk CF indicates that a potential exists to reduce GHG emissions from milk production on both national and farm level through changes in management. As milk yield and feed DMI are two of the most influential parameters for milk CF, feed conversion efficiency (i.e. units ECM produced per unit DMI) can be used as a rough key performance indicator for predicting CF reductions. However, it must be borne in mind that feeds have different CF due to where and how they are produced
Critical fluctuations and anomalous transport in soft Yukawa-Langevin systems
Simulation of a Langevin-dynamics model demonstrates emergence of critical
fluctuations and anomalous grain transport which have been observed in
experiments on "soft" quasi-two-dimensional dusty plasma clusters. It has been
suggested that these anomalies derive from particular non-equilibrium physics,
but our model does not contain such physics: the grains are confined by an
external potential, interact via static Yukawa forces, and are subject to
stochastic heating and dissipation from neutrals. One remarkable feature is
emergence of leptokurtic probability distributions of grain displacements
on time-scales , where is the
time at which the standard deviation
approaches the mean inter-grain distance . Others are development of
humps in the distributions on multiples of , anomalous Hurst exponents,
and transitions from leptokurtic towards Gaussian displacement distributions on
time scales . The latter is a signature of intermittency,
here interpreted as a transition from bursty transport associated with hopping
on intermediate time scales to vortical flows on longer time scales.Comment: 12 pages, 9 figure
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