43,544 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
Convergence of statistical moments of particle density time series in scrape-off layer plasmas
Particle density fluctuations in the scrape-off layer of magnetically
confined plasmas, as measured by gas-puff imaging or Langmuir probes, are
modeled as the realization of a stochastic process in which a superposition of
pulses with a fixed shape, an exponential distribution of waiting times and
amplitudes represents the radial motion of blob-like structures. With an
analytic formulation of the process at hand, we derive expressions for the
mean-squared error on estimators of sample mean and sample variance as a
function of sample length, sampling frequency, and the parameters of the
stochastic process. % Employing that the probability distribution function of a
particularly relevant shot noise process is given by the gamma distribution, we
derive estimators for sample skewness and kurtosis, and expressions for the
mean-squared error on these estimators.
Numerically generated synthetic time series are used to verify the proposed
estimators, the sample length dependency of their mean-squared errors, and
their performance.
We find that estimators for sample skewness and kurtosis based on the gamma
distribution are more precise and more accurate than common estimators based on
the method of moments.Comment: 31 pages, 10 figure
The spectroscopic orbits and the geometrical configuration of the symbiotic binary AR Pavonis
We analyze optical and near infrared spectra of intermediate and high
resolution of the eclipsing symbiotic system AR Pavonis. We have obtained the
radial velocity curves for the red and the hot component from the M-giant
absorption lines and from the wings of Halpha, H and He II4686 emission
profiles, respectively. From the orbital elements we have derived the masses,
Mgiant=2.5 and Mhot =1.0 solar masses, for the red giant and the hot component,
respectively. We also present and discuss radial velocity patterns in the blue
cF absorption spectrum as well as various emission lines. In particular, we
confirm that the blue absorption lines are associated with the hot component.
The radial velocity curve of the blue absorption system, however, does not
track the hot companion's orbital motion in a straightforward way, and its
departures from an expected circular orbit are particularly strong when the hot
component is active. We suggest that the cF-type absorption system is formed in
material streaming from the giant presumably in a region where the stream
encounters an accretion disk or an extended envelope around the hot component.
The broad emission wings originate from the inner accretion disk or the
envelope around the hot star.We also suggest that the central absorption in H
profiles is formed in a neutral portion of the cool giant's wind which is
strongly concentrated towards the orbital plane. The nebula in AR Pav seems to
be bounded by significant amount of neutral material in the orbital plane. The
forbidden emission lines are probably formed in low density ionized regions
extended in polar directions and/or the wind-wind interaction zone.Comment: 12 pages, 5 figures, accepted by A&
Stochastic modelling of intermittent scrape-off layer plasma fluctuations
Single-point measurements of fluctuations in the scrape-off layer of
magnetized plasmas are generally found to be dominated by large-amplitude
bursts which are associated with radial motion of blob-like structures. A
stochastic model for these fluctuations is presented, with the plasma density
given by a random sequence of bursts with a fixed wave form. Under very general
conditions, this model predicts a parabolic relation between the skewness and
kurtosis moments of the plasma fluctuations. In the case of exponentially
distributed burst amplitudes and waiting times, the probability density
function for the fluctuation amplitudes is shown to be a Gamma distribution
with the scale parameter given by the average burst amplitude and the shape
parameter given by the ratio of the burst duration and waiting times.Comment: 11 pages, 1 figur
Synchronization of Chaotic Systems by Common Random Forcing
We show two examples of noise--induced synchronization. We study a 1-d map
and the Lorenz systems, both in the chaotic region. For each system we give
numerical evidence that the addition of a (common) random noise, of large
enough intensity, to different trajectories which start from different initial
conditions, leads eventually to the perfect synchronization of the
trajectories. The largest Lyapunov exponent becomes negative due to the
presence of the noise terms.Comment: 5 pages, uses aipproc.cls and aipproc.sty (included). Five double
figures are provided as ten separate gif files. Version with (large)
postscript figures included available from
http://www.imedea.uib.es/PhysDept/publicationsDB/date.htm
The ages of very cool hydrogen-rich white dwarfs
The evolution of white dwarfs is essentially a cooling process that depends
primarily on the energy stored in their degenerate cores and on the
transparency of their envelopes. In this paper we compute accurate cooling
sequences for carbon-oxygen white dwarfs with hydrogen dominated atmospheres
for the full range of masses of interest. For this purpose we use the most
accurate available physical inputs for both the equation of state and opacities
of the envelope and for the thermodynamic quantities of the degenerate core. We
also investigate the role of the latent heat in the computed cooling sequences.
We present separately cooling sequences in which the effects of phase
separation of the carbon-oxygen binary mixture upon crystallization have been
neglected, and the delay introduced in the cooling times when this mechanism is
properly taken into account, in order to compare our results with other
published cooling sequences which do not include a treatment of this
phenomenon. We find that the cooling ages of very cool white dwarfs with pure
hydrogen atmospheres have been systematically underestimated by roughly 1.5 Gyr
at log(L/Lo)=-4.5 for an otherwise typical 0.6 Mo white dwarf, when phase
separation is neglected. If phase separation of the binary mixture is included
then the cooling ages are further increased by roughly 10%. Cooling tracks and
cooling isochrones in several color-magnitude diagrams are presented as well.Comment: 8 Pages; ApJ, accepted for publicatio
Intermittent fluctuations in the Alcator C-Mod scrape-off layer for ohmic and high confinement mode plasmas
Plasma fluctuations in the scrape-off layer of the Alcator C-Mod tokamak in
ohmic and high confinement modes have been analyzed using gas puff imaging
data. In all cases investigated, the time series of emission from a single
spatially-resolved view into the gas puff are dominated by large-amplitude
bursts, attributed to blob-like filament structures moving radially outwards
and poloidally. There is a remarkable similarity of the fluctuation statistics
in ohmic plasmas and in edge localized mode-free and enhanced D-alpha high
confinement mode plasmas. Conditionally averaged wave forms have a two-sided
exponential shape with comparable temporal scales and asymmetry, while the
burst amplitudes and the waiting times between them are exponentially
distributed. The probability density functions and the frequency power spectral
densities are self-similar for all these confinement modes. These results are
strong evidence in support of a stochastic model describing the plasma
fluctuations in the scrape-off layer as a super-position of uncorrelated
exponential pulses. Predictions of this model are in excellent agreement with
experimental measurements in both ohmic and high confinement mode plasmas. The
stochastic model thus provides a valuable tool for predicting
fluctuation-induced plasma-wall interactions in magnetically confined fusion
plasmas.Comment: 17 pages, 10 figure
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