2,022 research outputs found
Multi-layer study of wave propagation in sunspots
We analyze the propagation of waves in sunspots from the photosphere to the
chromosphere using time series of co-spatial Ca II H intensity spectra
(including its line blends) and polarimetric spectra of Si I 10827 and the He I
10830 multiplet. From the Doppler shifts of these lines we retrieve the
variation of the velocity along the line-of-sight at several heights. Phase
spectra are used to obtain the relation between the oscillatory signals. Our
analysis reveals standing waves at frequencies lower than 4 mHz and a
continuous propagation of waves at higher frequencies, which steepen into
shocks in the chromosphere when approaching the formation height of the Ca II H
core. The observed non-linearities are weaker in Ca II H than in He I lines.
Our analysis suggests that the Ca II H core forms at a lower height than the He
I 10830 line: a time delay of about 20 s is measured between the Doppler signal
detected at both wavelengths. We fit a model of linear slow magnetoacoustic
wave propagation in a stratified atmosphere with radiative losses according to
Newton's cooling law to the phase spectra and derive the difference in the
formation height of the spectral lines. We show that the linear model describes
well the wave propagation up to the formation height of Ca II H, where
non-linearities start to become very important.Comment: Accepted by The Astrophysical Journa
Light propagation control by finite-size effects in photonic crystals
We exhibit the strong influence on light propagation of the finite size in
photonic band-gap material. We show that light emission can be controlled by
the symmetry group of the boundary of the finite device. These results lead
simply to important practical applications.Comment: 5 pages, 4 figures, Revte
Application of demand flow technology to cable assembly production line
Includes bibliographical references
The Hanle and Zeeman Effects in Solar Spicules: A Novel Diagnostic Window on Chromospheric Magnetism
An attractive diagnostic tool for investigating the magnetism of the solar
chromosphere is the observation and theoretical modeling of the Hanle and
Zeeman effects in spicules, as shown in this letter for the first time. Here we
report on spectropolarimetric observations of solar chromospheric spicules in
the He I 10830 \AA multiplet and on their theoretical modeling accounting for
radiative transfer effects. We find that the magnetic field in the observed
(quiet Sun) spicular material at a height of about 2000 km above the visible
solar surface has a strength of the order of 10 G and is inclined by
approximately with respect to the local vertical direction. Our
empirical finding based on full Stokes-vector spectropolarimetry should be
taken into account in future magnetohydrodynamical simulations of spicules.Comment: 12 pages and 2 figure
Efficient light coupling from integrated single-mode waveguides to supercollimating photonic crystals on silicon-on-insulator platforms
We propose a practical and efficient solution for the coupling of light from
integrated single-mode waveguides to supercollimating planar photonic crystals
on conventional silicon-on-insulator platforms. The device consists of a rib
waveguide, designed to sustain spatially extended single-modes and matched to a
supercollimating photonic crystal, which has been truncated at its boundary to
improve impedance matching between the two photonic components.
Three-dimensional simulations show transmission efficiencies up to 96 % and
reflections below 0.2 % at wavelengths close to 1.55 microns. This approach
constitutes a significant step toward the integration of supercollimating
structures on photonic chips.Comment: 11 pages, 4 figure
REDUCED-SCALE STUDY OF LIQUID FUEL STORAGE TANK FIRE USING FIRE DYNAMICS SIMULATOR
Most of the accidents that occur in liquid fuel storage tank parks are caused by fire. This paper presents a numerical study using Large Eddy Simulation through Fire Dynamics Simulator (FDS) for the simulation of liquid fuel (ethanol) storage tanks at different scales (real-scale 1:1, and reduced- scales, 1:2, 1:4, 1:8). This paper proposes correlations for flame height, and temperature profile and radiative heat flux profile in the region adjacent to the tanks. Correlations have as inputs the diameters of the tanks in real- and reduced-scale, temperature profiles and radiative heat flux profiles for a reduced-scale tank simulation, and then provide as outputs flame height and temperature profiles and radiative heat flux profiles for the tank in real- scale. Percentage errors of the correlations found in this study are lower than 2.0% and 0.6% for the maximum radiative heat flux and maximum temperature, respectively
Numerical simulations of conversion to Alfven waves in sunspots
We study the conversion of fast magneto-acoustic waves to Alfven waves by
means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A
fast, essentially acoustic, wave of a given frequency and wave number is
generated below the surface and propagates upward though the Alfven/acoustic
equipartition layer where it splits into upgoing slow (acoustic) and fast
(magnetic) waves. The fast wave quickly reflects off the steep Alfven speed
gradient, but around and above this reflection height it partially converts to
Alfven waves, depending on the local relative inclinations of the background
magnetic field and the wavevector. To measure the efficiency of this conversion
to Alfven waves we calculate acoustic and magnetic energy fluxes. The
particular amplitude and phase relations between the magnetic field and
velocity oscillations help us to demonstrate that the waves produced are indeed
Alfven waves. We find that the conversion to Alfven waves is particularly
important for strongly inclined fields like those existing in sunspot
penumbrae. Equally important is the magnetic field orientation with respect to
the vertical plane of wave propagation, which we refer to as "field azimuth".
For field azimuth less than 90 degrees the generated Alfven waves continue
upwards, but above 90 degrees downgoing Alfven waves are preferentially
produced. This yields negative Alfven energy flux for azimuths between 90 and
180 degrees. Alfven energy fluxes may be comparable to or exceed acoustic
fluxes, depending upon geometry, though computational exigencies limit their
magnitude in our simulations.Comment: Accepted for publication in Ap
Bonus systems in an open portfolio
In this paper, we study bonus systems in an open portfolio, i.e. we consider that a policyholder can transfer his policy to a different insurance company at any time. We make use of inhomogeneous Markov chains to model the system and show, under reasonable assumptions, that the stationary distribution is independent of the market shares, and is easily calculated.info:eu-repo/semantics/publishedVersio
Ratemaking of dependent risks
We start by describing how, in some cases, we can use variance-related premium principles in ratemaking, when the claim numbers and individual claim amounts are independent. We use quasi-likelihood generalized linear models, under the assumption that the variance function is a power function of the mean of the underlying random variable. We extend this approach to the cases where the claim numbers are correlated. Some alternatives to deal with dependent risks are presented, taking explicitly into account overdispersion. We present regression models covering the bivariate Poisson, the generalized bivariate negative binomial and the bivariate Poisson–Laguerre polynomial, which nest the bivariate negative binomial. We apply these models to a portfolio of the Portuguese insurance company Tranquilidade and compare the results obtained.info:eu-repo/semantics/publishedVersio
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