305 research outputs found
When do finite sample effects significantly affect entropy estimates ?
An expression is proposed for determining the error caused on entropy
estimates by finite sample effects. This expression is based on the Ansatz that
the ranked distribution of probabilities tends to follow an empirical Zipf law.Comment: 10 pages, 2 figure
A method for filling gaps in solar irradiance and in solar proxy data
Data gaps are ubiquitous in spectral irradiance data, and yet, little effort
has been put into finding robust methods for filling them. We introduce a
data-adaptive and nonparametric method that allows us to fill data gaps in
multi-wavelength or in multichannel records. This method, which is based on the
iterative singular value decomposition, uses the coherency between simultaneous
measurements at different wavelengths (or between different proxies) to fill
the missing data in a self-consistent way. The interpolation is improved by
handling different time scales separately.
Two major assets of this method are its simplicity, with few tuneable
parameters, and its robustness. Two examples of missing data are given: one
from solar EUV observations, and one from solar proxy data. The method is also
appropriate for building a composite out of partly overlapping records.Comment: to appear in Astronomy & Astrophysics (2011
Non-Gaussian statistics in space plasma turbulence, fractal properties and pitfalls
Magnetic field fluctuations in the vicinity of the Earth's bow shock have
been investigated with the aim to characterize the intermittent behaviour of
strong plasma turbulence. The observed small-scale intermittency may be the
signature of a multifractal process but a deeper inspection reveals caveats in
such an interpretation. Several effects, including the anisotropy of the
wavefield, the violation of the Taylor hypothesis and the occasional occurrence
of coherent wave packets, strongly affect the higher order statistical
properties. After correcting these effects, a more Gaussian and scale-invariant
wavefield is recovered.Comment: 13 pages (including 13 postscript figures), to appear in Nonlinear
Processes in Geophysic
La décomposition en modes propres : une approche nouvelle pour caractériser le confinement du plasma dans un tokamak
Determination of the most pertinent EUV proxy for use in thermosphere modeling
Two major issues in the specification of the thermospheric density are the
definition of proper solar inputs and the empirical modeling of thermosphere
response to solar and to geomagnetic forcings. This specification is crucial
for the tracking of low Earth orbiting satellites.
Here we address both issues by using 14 years of daily density measurements
made by the Stella satellite at 813 km altitude and by carrying out a
multiscale statistical analysis of various solar inputs. First, we find that
the spectrally integrated solar emission between 26-34 nm offers the best
overall performance in the density reconstruction. Second, we introduce linear
parametric transfer function models to describe the dynamic response of the
density to the solar and geomagnetic forcings. These transfer function models
lead to a major error reduction and in addition open new perspectives in the
physical interpretation of the thermospheric dynamics
Methods for characterising microphysical processes in plasmas
Advanced spectral and statistical data analysis techniques have greatly
contributed to shaping our understanding of microphysical processes in plasmas.
We review some of the main techniques that allow for characterising fluctuation
phenomena in geospace and in laboratory plasma observations. Special emphasis
is given to the commonalities between different disciplines, which have
witnessed the development of similar tools, often with differing terminologies.
The review is phrased in terms of few important concepts: self-similarity,
deviation from self-similarity (i.e. intermittency and coherent structures),
wave-turbulence, and anomalous transport.Comment: Space Science Reviews (2013), in pres
The frequency splitting of the axisymmetric magnetoacoustic wave as a current profile diagnostic
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