706 research outputs found
Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope
Aims: We aim to achieve high spatial resolution as well as high polarimetric
sensitivity, using an earth-based 1m-class solar telescope, for the study of
magnetic fine structure on the Sun. Methods: We use a setup with 3 high-speed,
low-noise cameras to construct datasets with interleaved polarimetric states,
particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image
restorations. We discuss the polarimetric calibration routine as well as
various potential sources of error in the results. Results: We obtained near
diffraction limited images, with a noise level of approximately 10^(-3)
I(cont). We confirm that dark-cores have a weaker magnetic field and at a lower
inclination angle with respect to the solar surface than the edges of the
penumbral filament. We show that the magnetic field strength in
faculae-striations is significantly lower than in other nearby parts of the
faculae.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 11
figure
Fine structure, magnetic field and heating of sunspot penumbrae
We interpret penumbral filaments as due to convection in field-free, radially
aligned gaps just below the visible surface of the penumbra, intruding into a
nearly potential field above. This solves the classical discrepancy between the
large heat flux and the low vertical velocities observed in the penumbra. The
presence of the gaps causes strong small-scale fluctuations in inclination,
azimuth angle and field strength, but without strong forces acting on the gas.
The field is nearly horizontal in a region around the cusp-shaped top of the
gap, thereby providing an environment for Evershed flows. We identify this
region with the recently discovered dark penumbral cores. Its darkness has the
same cause as the dark lanes in umbral light-bridges, reproduced in numerical
simulations by Nordlund and Stein (2005). We predict that the large vertical
and horizontal gradients of the magnetic field inclination and azimuth in the
potential field model will produce the net circular polarization seen in
observations. The model also explains the significant elevation of bright
filaments above their surroundings. It predicts that dark areas in the penumbra
are of two different kinds: dark filament cores containing the most inclined
(horizontal) fields, and regions between bright filaments, containing the least
inclined field lines.Comment: submitted to A&
The Effects of Atmospheric Dispersion on High-Resolution Solar Spectroscopy
We investigate the effects of atmospheric dispersion on observations of the
Sun at the ever-higher spatial resolutions afforded by increased apertures and
improved techniques. The problems induced by atmospheric refraction are
particularly significant for solar physics because the Sun is often best
observed at low elevations, and the effect of the image displacement is not
merely a loss of efficiency, but the mixing of information originating from
different points on the solar surface. We calculate the magnitude of the
atmospheric dispersion for the Sun during the year and examine the problems
produced by this dispersion in both spectrographic and filter observations. We
describe an observing technique for scanning spectrograph observations that
minimizes the effects of the atmospheric dispersion while maintaining a regular
scanning geometry. Such an approach could be useful for the new class of
high-resolution solar spectrographs, such as SPINOR, POLIS, TRIPPEL, and ViSP
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Turbulence Fluctuations and New Universal Realizability Conditions in Modelling
General turbulent mean statistics are shown to be characterized by a
variational principle. The variational functionals, or ``effective actions'',
have experimental consequences for turbulence fluctuations and are subject to
realizability conditions of positivity and convexity. An efficient
Rayleigh-Ritz algorithm is available to calculate approximate effective actions
within PDF closures. Examples are given for Navier-Stokes and for a 3-mode
system of Lorenz. The new realizability conditions succeed at detecting {\em a
priori} the poor predictions of PDF closures even when the classical 2nd-order
moment realizability conditions are satisfied.Comment: 4 pages, LaTeX (Version 2.09), 3 figures, Postscript, Submitted to
Phys. Rev. Let
Fluctuations in the Irreversible Decay of Turbulent Energy
A fluctuation law of the energy in freely-decaying, homogeneous and isotropic
turbulence is derived within standard closure hypotheses for 3D incompressible
flow. In particular, a fluctuation-dissipation relation is derived which
relates the strength of a stochastic backscatter term in the energy decay
equation to the mean of the energy dissipation rate. The theory is based on the
so-called ``effective action'' of the energy history and illustrates a
Rayleigh-Ritz method recently developed to evaluate the effective action
approximately within probability density-function (PDF) closures. These
effective actions generalize the Onsager-Machlup action of nonequilibrium
statistical mechanics to turbulent flow. They yield detailed, concrete
predictions for fluctuations, such as multi-time correlation functions of
arbitrary order, which cannot be obtained by direct PDF methods. They also
characterize the mean histories by a variational principle.Comment: 26 pages, Latex Version 2.09, plus seceq.sty, a stylefile for
sequential numbering of equations by section. This version includes new
discussion of the physical interpretation of the formal Rayleigh-Ritz
approximation. The title is also change
Decay laws for three-dimensional magnetohydrodynamic turbulence
Decay laws for three-dimensional magnetohydrodynamic turbulence are obtained
from high-resolution numerical simulations using up to 512^3 modes...
Lac Blanc Pass: a natural wind-tunnel for studying drifting snow at 2700ma.s.l
International audienceThe investigation of the spatial variability of snow depth in high alpine areas is an important topic in snow hydrology, glacier and avalanche research and the transport of snow by wind is an important process for the distribution of snow in mountainous regions. That's why, for 25 years IRSTEA (previously Cemagref) and Météo France (Centre for the Study of Snow) have joined together in studying drifting snow at Col du Lac Blanc 2700 m a.s.l. near the Alpe d'Huez ski resort in the French Alps. Initially, the site was mainly equipped with conventional meteorological stations and a network of snow poles, in order to test numerical models of drifting snow Sytron (CEN) and NEMO (Cemagref). These models are complementary in terms of spatial and temporal scales: outputs of Sytron model will form the inputs of NEMO model. Then new sensors and technologies appeared which allow to develop new knowledge dealing with thresholds velocity according to morphological features of snow grains, snow flux profiles including parameters such as fall velocity and Schmidt number, histograms of particle widths, aerodynamic roughness, gust factors. More recently, the coupled snowpack/ atmosphere model Meso-NH/Crocus has been evaluated at the experimental site. At the same time, some tested sensors have been deployed in Adelie Land in Antarctica, where blowing snow accounts for a major component of the surface mass balance. Japanese and Austrian research teams have been accomodated at Lac Blanc Pass and new foreign teams are welcome. Initial observations continue. That's why Lac Blanc Pass is also a climatological reference for 25 years at 2700 m. Data are available
Separation between coherent and turbulent fluctuations. What can we learn from the Empirical Mode Decomposition?
The performances of a new data processing technique, namely the Empirical
Mode Decomposition, are evaluated on a fully developed turbulent velocity
signal perturbed by a numerical forcing which mimics a long-period flapping.
First, we introduce a "resemblance" criterion to discriminate between the
polluted and the unpolluted modes extracted from the perturbed velocity signal
by means of the Empirical Mode Decomposition algorithm. A rejection procedure,
playing, somehow, the role of a high-pass filter, is then designed in order to
infer the original velocity signal from the perturbed one. The quality of this
recovering procedure is extensively evaluated in the case of a "mono-component"
perturbation (sine wave) by varying both the amplitude and the frequency of the
perturbation. An excellent agreement between the recovered and the reference
velocity signals is found, even though some discrepancies are observed when the
perturbation frequency overlaps the frequency range corresponding to the
energy-containing eddies as emphasized by both the energy spectrum and the
structure functions. Finally, our recovering procedure is successfully
performed on a time-dependent perturbation (linear chirp) covering a broad
range of frequencies.Comment: 23 pages, 13 figures, submitted to Experiments in Fluid
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