81,703 research outputs found
Polarized light scattering by aerosols in the marine atmospheric boundary layer
The intensity and polarization of light scattered from marine aerosols affect visibility and contrast in the marine atmospheric boundary layer (MABL). The polarization properties of scattered light in the MABL vary with size, refractive index, number distributions, and environmental conditions. Laboratory measurements were used to determine the characteristics and variability of the polarization of light scattered by aerosols similar to those in the MABL. Scattering from laboratory-generated sea-salt-containing (SSC) [NaCl, (NH4)2SO4, and seawater] components of marine aerosols was measured with a scanning polarization-modulated nephelometer. Mie theory with Gaussian and log normal size distributions of spheres was used to calculate the polarized light scattering from various aerosol composition models and from experimentally determined distributions of aerosols in the marine boundary layer. The modeling was verified by comparison with scattering from distilled water aerosols. The study suggests that polarimetric techniques can be used to enhance techniques for improving visibility and remote imaging for various aerosol types, Sun angles, and viewing conditions
Some Considerations about Geometric Algebras in relation with Visibility in Computer Graphics
We give some considerations about the use of geometric algebras in the context of visibility, showing some advantages and disadvantages for their use as the underlying framework. We emphasize the use of conformal geometric algebra since, among other reasons, it allows us to study easily the visibility for flat varieties and, due to the same algebraic expression of hyper-spheres and linear varieties, the results might be generalized to non-flat objects
On using visibility correlations to probe the HI distribution from the dark ages to the present epoch I: Formalism and the expected signal
Redshifted 21 cm radiation originating from the cosmological distribution of
neutral hydrogen (HI) appears as a background radiation in low frequency radio
observations. The angular and frequency domain fluctuations in this radiation
carry information about cosmological structure formation. We propose that
correlations between visibilities measured at different baselines and
frequencies in radio-interferometric observations be used to quantify the
statistical properties of these fluctuations. This has an inherent advantage
over other statistical estimators in that it deals directly with the
visibilities which are the primary quantities measured in radio-interferometric
observations. Also, the visibility correlation has a very simple relation with
power spectrum. We present estimates of the expected signal for nearly the
entire post-recombination era, from the dark ages to the present epoch. The
epoch of reionization, where the HI has a patchy distribution, has a distinct
signature where the signal is determined by the size of the discrete ionized
regions. The signal at other epochs, where the HI follows the dark matter, is
determined largely by the power spectrum of dark matter fluctuations. The
signal is strongest for baselines where the antenna separations are within a
few hundred times the wavelength of observation, and an optimal strategy would
preferentially sample these baselines. In the frequency domain, for most
baselines the visibilities at two different frequencies are uncorrelated beyond
\Delta \nu ~ 1 MHz, a signature which in principle would allow the HI signal to
be easily distinguished from the continuum sources of contamination.Comment: 12 pages, 9 figures, Accepted to MNRAS; Replaced to match version
accepted in MNRA
Blind spheres of paramagnetic dopants in solid state NMR
Solid-state NMR on paramagnetically doped crystal structures gives information about the spatial distribution of dopants in the host. Paramagnetic dopants may render NMR active nuclei virtually invisible by relaxation, paramagnetic broadening or shielding. In this contribution blind sphere radii r(0) have been reported, which could be extracted through fitting the NMR signal visibility function f (x) = exp(-ar(0)(3)x) to experimental data obtained on several model compound series: La(1-x)Ln(x)PO(4) (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb), Sr1-xEuxGa2S4 and (Zn1-xMnx)(3)(PO4)(2)center dot 4H(2)O. Radii were extracted for H-1, P-31 and Ga-71, and dopants like Nd3+, Gd3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Mn2+. The observed radii determined differed in all cases and covered a range from 5.5 to 13.5 angstrom. While these radii were obtained from the amount of invisible NMR signal, we also show how to link the visibility function to lineshape parameters. We show under which conditions empirical correlations of linewidth and doping concentration can be used to extract blind sphere radii from second moment or linewidth parameter data. From the second moment analysis of La1-xSmxPO4 P-31 MAS NMR spectra for example, a blind sphere size of Sm3+ can be determined, even though the visibility function remains close to 100% over the entire doping range. Dependence of the blind sphere radius r(0) on the NMR isotope and on the paramagnetic dopant could be suggested and verified: for different nuclei, r(0) shows a 3 root gamma-dependence, gamma being the gyromagnetic ratio. The blind sphere radii r(0) for different paramagnetic dopants in a lanthanide series could be predicted from the pseudo-contact term
Observation of Entanglement-Dependent Two-Particle Holonomic Phase
Holonomic phases---geometric and topological---have long been an intriguing
aspect of physics. They are ubiquitous, ranging from observations in particle
physics to applications in fault tolerant quantum computing. However, their
exploration in particles sharing genuine quantum correlations lack in
observations. Here we experimentally demonstrate the holonomic phase of two
entangled-photons evolving locally, which nevertheless gives rise to an
entanglement-dependent phase. We observe its transition from geometric to
topological as the entanglement between the particles is tuned from zero to
maximal, and find this phase to behave more resilient to evolution changes with
increasing entanglement. Furthermore, we theoretically show that holonomic
phases can directly quantify the amount of quantum correlations between the two
particles. Our results open up a new avenue for observations of holonomic
phenomena in multi-particle entangled quantum systems.Comment: 8 pages, 6 figure
Doping homogeneity in co-doped materials investigated at different length scales
Doping homogeneity is important for the properties of co-doped phosphors, as it can affect the energy transfer between sensitizer and activator ions. In a case study we apply different methods, that is
scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX)
mapping, SEM combined with cathodoluminescence (CL) and solid-state nuclear magnetic resonance
(NMR), to study the doping homogeneity of the host system monazite LaPO4 doped with two different
lanthanide ions on different length scales. A new criterion for doping heterogeneity in co-doped
systems is developed, which is based on the NMR visibility function, which for this purpose is extended
to doping with two or more paramagnetic dopants. A deviation from this function is indicative of doping
heterogeneity on the length-scale of the blind-spheres of the paramagnetic dopants. A discussion of the
advantages and disadvantages of the different methods is presented. The combined approach allows to
study doping homogeneity from the nm to the mm scale
Is the shell-focusing singularity of Szekeres space-time visible?
The visibility of the shell-focusing singularity in Szekeres space-time—which represents quasispherical dust collapse—has been studied on numerous occasions in the context of the cosmic censorship conjecture. The various results derived have assumed that there exist radial null geodesics in the space-time. We show that such geodesics do not exist in general, and so previous results on the visibility of the singularity are not generally valid. More precisely, we show that the existence of a radial geodesic in Szekeres space-time implies that the space-time is axially symmetric, with the geodesic along the polar direction (i.e. along the axis of symmetry). If there is a second nonparallel radial geodesic, then the space-time is spherically symmetric, and so is a Lemaître-Tolman-Bondi space-time. For the case of the polar geodesic in an axially symmetric Szekeres space-time, we give conditions on the free functions (i.e. initial data) of the space-time which lead to visibility of the singularity along this direction. Likewise, we give a sufficient condition for censorship of the singularity. We point out the complications involved in addressing the question of visibility of the singularity both for nonradial null geodesics in the axially symmetric case and in the general (nonaxially symmetric) case, and suggest a possible approach
From Sochi - 2014 to FIFA - 2018: a Fading Sovereignty?
In this article, we uncover the dynamics and the evolution of Russian discourses of sovereignty before and after the Sochi 2014 Olympic Games using some elements of Foucauldian methodology and constructivist reading of sovereignty as an institution. We argue that there is a discrepancy between the rhetoric of sovereign power and the institutional practices in which it is embedded. It leads us to theorize that sovereignty discourses are contextual, unstable and constitutively shaped by commitments taken as key elements of international socialization. In the case of Russia, these discourses can be divided into three groups: pre-Sochi, post-Sochi and pre-World 2018 Cup discursive formations. As we venture to demonstrate, Putin's model of sovereignty is in crisis, yet it has support, both domestic and international. In the near future, sport is likely to remain one of those spheres of high visibility where the ideology of surviving under sanctions and counter-attacking the West will be reified
Is the shell-focusing singularity of Szekeres space-time visible?
The visibility of the shell-focusing singularity in Szekeres space-time -
which represents quasi-spherical dust collapse - has been studied on numerous
occasions in the context of the cosmic censorship conjecture. The various
results derived have assumed that there exist radial null geodesics in the
space-time. We show that such geodesics do not exist in general, and so
previous results on the visibility of the singularity are not generally valid.
More precisely, we show that the existence of a radial geodesic in Szekeres
space-time implies that the space-time is axially symmetric, with the geodesic
along the polar direction (i.e. along the axis of symmetry). If there is a
second non-parallel radial geodesic, then the space-time is spherically
symmetric, and so is a Lema\^{\i}tre-Tolman-Bondi (LTB) space-time. For the
case of the polar geodesic in an axially symmetric Szekeres space-time, we give
conditions on the free functions (i.e. initial data) of the space-time which
lead to visibility of the singularity along this direction. Likewise, we give a
sufficient condition for censorship of the singularity. We point out the
complications involved in addressing the question of visibility of the
singularity both for non-radial null geodesics in the axially symmetric case
and in the general (non-axially symmetric) case, and suggest a possible
approach.Comment: 10 page
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