272 research outputs found
Grenada’s Culture and Traditional Dress
Grenada is a Caribbean island that is located in the continent of South America. Grenada, also known as the ‘Spice Isle’ has a population of 105,897. It is divided into seven parishes – St. George’s being the capital. The country's main cultural traditions were handed down by African ancestors, however other ethnic groups such as Indians and Europeans also made contributions. Due to the large presence of Africans, their traditions were the most influential and present-day traditional dress and cultural practices reflect their elements. Key Words: Grenada Culture, Carnival, Grenada’s Traditional Wea
The State of Self-Organized Criticality of the Sun During the Last Three Solar Cycles. II. Theoretical Model
The observed powerlaw distributions of solar flare parameters can be
interpreted in terms of a nonlinear dissipative system in the state of
self-organized criticality (SOC). We present a universal analytical model of a
SOC process that is governed by three conditions: (i) a multiplicative or
exponential growth phase, (ii) a randomly interrupted termination of the growth
phase, and (iii) a linear decay phase. This basic concept approximately
reproduces the observed frequency distributions. We generalize it to a
randomized exponential-growth model, which includes also a (log-normal)
distribution of threshold energies before the instability starts, as well as
randomized decay times, which can reproduce both the observed occurrence
frequency distributions and the scatter of correlated parametyers more
realistically. With this analytical model we can efficiently perform
Monte-Carlo simulations of frequency distributions and parameter correlations
of SOC processes, which are simpler and faster than the iterative simulations
of cellular automaton models. Solar cycle modulations of the powerlaw slopes of
flare frequency distributions can be used to diagnose the thresholds and growth
rates of magnetic instabilities responsible for solar flares.Comment: Part II of Paper I: The State of Self-Organized Criticality of the
Sun During the Last Three Solar Cycles. I. Observation
The open string pair-production rate enhancement by a magnetic flux
We extend the amplitude calculations of \cite{Lu:2009yx} to exhaust the
remaining cases for which one set of D branes carrying a flux (electric or
magnetic) is placed parallel at separation to the other set carrying also a
flux but with the two fluxes sharing at most one common field-strength index.
We then find that the basic structure of amplitudes remains the same when the
two fluxes share at least one common index but it is more general when the two
fluxes share no common index. We discuss various properties of the amplitudes
such as the large separation limit, the onset of various instabilities and the
open string pair production. In particular, when one flux is electric and weak
and the other is magnetic and fixed, we find that the open string pair
production rate is greatly enhanced by the presence of this magnetic flux when
the two fluxes share no common field-strength index and this rate becomes
significant when the separation is on the order of string scale.Comment: 33 pages, no figures, a few points refined to the published version
JHEP09(2009)09
Deterministically Driven Avalanche Models of Solar Flares
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting of a
coronal loop. They share similarities with other avalanche models, such as the
classical stick--slip self-organized critical model of earthquakes, in that
they are driven globally by a fully deterministic energy loading process. The
model design leads to a systematic deficit of small scale avalanches. In some
portions of model space, mid-size and large avalanching behavior is scale-free,
being characterized by event size distributions that have the form of
power-laws with index values, which, in some parameter regimes, compare
favorably to those inferred from solar EUV and X-ray flare data. For models
using conservative or near-conservative redistribution rules, a population of
large, quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy release,
this latter behavior may be relevant to recurrent flaring in individual coronal
loops. This class of models could provide a basis for the prediction of large
solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar
Physic
Time-dependent Stochastic Modeling of Solar Active Region Energy
A time-dependent model for the energy of a flaring solar active region is
presented based on a stochastic jump-transition model (Wheatland and Glukhov
1998; Wheatland 2008; Wheatland 2009). The magnetic free energy of the model
active region varies in time due to a prescribed (deterministic) rate of energy
input and prescribed (random) flare jumps downwards in energy. The model has
been shown to reproduce observed flare statistics, for specific
time-independent choices for the energy input and flare transition rates.
However, many solar active regions exhibit time variation in flare
productivity, as exemplified by NOAA active region AR 11029 (Wheatland 2010).
In this case a time-dependent model is needed. Time variation is incorporated
for two cases: 1. a step change in the rates of flare jumps; and 2. a step
change in the rate of energy supply to the system. Analytic arguments are
presented describing the qualitative behavior of the system in the two cases.
In each case the system adjusts by shifting to a new stationary state over a
relaxation time which is estimated analytically. The new model retains
flare-like event statistics. In each case the frequency-energy distribution is
a power law for flare energies less than a time-dependent rollover set by the
largest energy the system is likely to attain at a given time. For Case 1, the
model exhibits a double exponential waiting-time distribution, corresponding to
flaring at a constant mean rate during two intervals (before and after the step
change), if the average energy of the system is large. For Case 2 the
waiting-time distribution is a simple exponential, again provided the average
energy of the system is large. Monte Carlo simulations of Case~1 are presented
which confirm the analytic estimates. The simulation results provide a
qualitative model for observed flare statistics in active region AR 11029.Comment: 25 pages, 9 figure
The State of Self-Organized Criticality of the Sun During the Last 3 Solar Cycles. I. Observations
We analyze the occurrence frequency distributions of peak fluxes , total
fluxes , and durations of solar flares over the last three solar cycles
(during 1980--2010) from hard X-ray data of HXRBS/SMM, BATSE/CGRO, and RHESSI.
From the synthesized data we find powerlaw slopes with mean values of
for the peak flux, for the total
flux, and for flare durations. We find a systematic
anti-correlation of the powerlaw slope of peak fluxes as a function of the
solar cycle, varying with an approximate sinusoidal variation
, with a
mean of , a variation of , a solar cycle
period yrs, and a cycle minimum time . The
powerlaw slope is flattest during the maximum of a solar cycle, which indicates
a higher magnetic complexity of the solar corona that leads to an
overproportional rate of powerful flares.Comment: subm. to Solar Physic
On Four-Point Functions of Half-BPS Operators in General Dimensions
We study four-point correlation functions of half-BPS operators of arbitrary
weight for all dimensions d=3,4,5,6 where superconformal theories exist. Using
harmonic superspace techniques, we derive the superconformal Ward identities
for these correlators and present them in a universal form. We then solve these
identities, employing Jack polynomial expansions. We show that the general
solution is parameterized by a set of arbitrary two-variable functions, with
the exception of the case d=4, where in addition functions of a single variable
appear. We also discuss the operator product expansion using recent results on
conformal partial wave amplitudes in arbitrary dimension.Comment: The discussion of the case d=6 expanded; references added/correcte
Modeling the non-Markovian, non-stationary scaling dynamics of financial markets
A central problem of Quantitative Finance is that of formulating a
probabilistic model of the time evolution of asset prices allowing reliable
predictions on their future volatility. As in several natural phenomena, the
predictions of such a model must be compared with the data of a single process
realization in our records. In order to give statistical significance to such a
comparison, assumptions of stationarity for some quantities extracted from the
single historical time series, like the distribution of the returns over a
given time interval, cannot be avoided. Such assumptions entail the risk of
masking or misrepresenting non-stationarities of the underlying process, and of
giving an incorrect account of its correlations. Here we overcome this
difficulty by showing that five years of daily Euro/US-Dollar trading records
in the about three hours following the New York market opening, provide a rich
enough ensemble of histories. The statistics of this ensemble allows to propose
and test an adequate model of the stochastic process driving the exchange rate.
This turns out to be a non-Markovian, self-similar process with non-stationary
returns. The empirical ensemble correlators are in agreement with the
predictions of this model, which is constructed on the basis of the
time-inhomogeneous, anomalous scaling obeyed by the return distribution.Comment: Throughout revision. 15 pages, 6 figures. Presented by A.L. Stella in
a Talk at the "Econophysics - Kolkata V'' International Workshop, March 2010,
Saha Institute of Nuclear Physics, Kolkata, Indi
A Systematic Study on Energy Dependence of Quasi-Periodic Oscillation Frequency in GRS 1915+105
Systematically studying all the RXTE/PCA observations for GRS 1915+105 before
November 2010, we have discovered three additional patterns in the relation
between Quasi-Periodic Oscillation (QPO) frequency and photon energy, extending
earlier outcomes reported by Qu et al. (2010). We have confirmed that as QPO
frequency increases, the relation evolves from the negative correlation to
positive one. The newly discovered patterns provide new constraints on the QPO
models
Initial clinical evaluation of stationary digital chest tomosynthesis in adult patients with cystic fibrosis
Objective: The imaging evaluation of cystic fibrosis currently relies on chest radiography or computed tomography. Recently, digital chest tomosynthesis has been proposed as an alternative. We have developed a stationary digital chest tomosynthesis (s-DCT) system based on a carbon nanotube (CNT) linear x-ray source array. This system enables tomographic imaging without movement of the x-ray tube and allows for physiological gating. The goal of this study was to evaluate the feasibility of clinical CF imaging with the s-DCT system. Materials and methods: CF patients undergoing clinically indicated chest radiography were recruited for the study and imaged on the s-DCT system. Three board-certified radiologists reviewed both the CXR and s-DCT images for image quality relevant to CF. CF disease severity was assessed by Brasfield score on CXR and chest tomosynthesis score on s-DCT. Disease severity measures were also evaluated against subject pulmonary function tests. Results: Fourteen patients underwent s-DCT imaging within 72 h of their chest radiograph imaging. Readers scored the visualization of proximal bronchi, small airways and vascular pattern higher on s-DCT than CXR. Correlation between the averaged Brasfield score and averaged tomosynthesis disease severity score for CF was -0.73, p = 0.0033. The CF disease severity score system for tomosynthesis had high correlation with FEV1 (r = -0.685) and FEF 25–75% (r = -0.719) as well as good correlation with FVC (r = -0.582). Conclusion: We demonstrate the potential of CNT x-ray-based s-DCT for use in the evaluation of cystic fibrosis disease status in the first clinical study of s-DCT. Key Points: • Carbon nanotube-based linear array x-ray tomosynthesis systems have the potential to provide diagnostically relevant information for patients with cystic fibrosis without the need for a moving gantry. • Despite the short angular span in this prototype system, lung features such as the proximal bronchi, small airways and pulmonary vasculature have improved visualization on s-DCT compared with CXR. Further improvements are anticipated with longer linear x-ray array tubes. • Evaluation of disease severity in CF patients is possible with s-DCT, yielding improved visualization of important lung features and high correlation with pulmonary function tests at a relatively low dose
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