1,002 research outputs found
Resonantly enhanced filamentation in gases
In this Letter, a low-loss Kerr-driven optical filament in Krypton gas is
experimentally reported in the ultraviolet. The experimental findings are
supported by ab initio quantum calculations describing the atomic optical
response. Higher-order Kerr effect induced by three-photon resonant transitions
is identified as the underlying physical mechanism responsible for the
intensity stabilization during the filamentation process, while ionization
plays only a minor role. This result goes beyond the commonly-admitted paradigm
of filamentation, in which ionization is a necessary condition of the filament
intensity clamping. At resonance, it is also experimentally demonstrated that
the filament length is greatly extended because of a strong decrease of the
optical losses
Environmental control on coccolithophore morphology : do modern species yield information that is transferable to the geological past?
It is generally assumed that calcareous nannofossils conserve palaeoenvironmental information from the time of their formation. Changes in coccolith morphology can result from physiological responses to environmental drivers. Temperature, salinity, nutrient concentration, light and carbonate chemistry are among the environmental drivers that impact extant coccolithophores and may alter coccolith size, as well as coccosphere size and morphology. Many palaeoreconstruction studies have assessed the biological responses of living coccolithophore species to environmental drivers with the expectation that it is possible to use this information for calibrating the biomineralisation responses of ancient coccolithophores. However, there is a large uncertainty concerning whether the morphological responses of living coccolithophores to environmental changes are similar to the morphological responses of fossil species, when you consider the fact that millions of years of evolutionary adaptation lie between the extant species and their fossilised ancestors. In order to test this caveat, we examined four extant species (Emiliania huxleyi, Gephyrocapsa oceanica , Coccolithus pelagicus subsp. braarudii and Pleurochrysis carterae), which have been evolutionarily distinct for millions of years. We cultured them under changing environmental conditions in order to evaluate any changes in coccolith morphology. Our underlying hypothesis was that if the species showed a uniform reaction to any of the tested environmental drivers, then this would suggests that the same response may well occur over geological timescales, and that coccolith morphological changes could serve as a palaeo-proxy for that particular driver. Our experiments demonstrated that the four species had no common response to changing light intensity, Mg/Ca, nutrient content or temperature with respect to coccolith size. These results revealed the difficulties in using coccolith size as a proxy for environmental drivers. One exception was an increase in malformations when coccolithophores were grown under excess CO2 , and these data provided evidence that this response variable can be used as a palaeo-proxy for episodes of acute carbonate chemistry perturbations
Orientation and Alignment Echoes
We present what is probably the simplest classical system featuring the echo
phenomenon - a collection of randomly oriented free rotors with dispersed
rotational velocities. Following excitation by a pair of time-delayed impulsive
kicks, the mean orientation/alignment of the ensemble exhibits multiple echoes
and fractional echoes. We elucidate the mechanism of the echo formation by
kick-induced filamentation of phase space, and provide the first experimental
demonstration of classical alignment echoes in a thermal gas of CO_2 molecules
excited by a pair of femtosecond laser pulses
Fluid-structure interaction and homogenization: from spatial averaging to continuous wavelet transform
Fluid-structure interaction (FSI) is classicaly modeled according a separated and local approach. It enables to take full advantage of the numerical methods specifically designed for each medium. However, it requires to take great care of the interface, and to exchange, between the algorithms, the information related to boundary conditions [1]. This treatment of the interface can quickly become too cumbersome in complex flow geometries, as in the industrial case study driving this work: an inviscid compressible flow interacting with French PWR fuel assemblies (Fig. 1a).
In such specific applications, where the solid medium exhibits a discontinuous but periodic design, an homogenized and global approach is preferred [2]. Inspired by porous media [3, 4], multiphase flows, or Large Eddy Simulation (LES), it relies on a spatial averaging of the balance equations, thus allowing to remove all interfaces. However, such filtering techniques exhibit two major limitations: first, they do not deal properly with boundary conditions, due to the non-commutativity between the filtering operator and spatial derivatives, as detailed in [5, 6, 7] for LES; second, filtering implies loss of microscopic information, and thus requires a closure model to describe interactions between resolved and unresolved scales
High resolution spatial analyses of trace elements in coccoliths reveal new insights into element incorporation in coccolithophore calcite
The Line-of-Sight Proximity Effect and the Mass of Quasar Host Halos
We show that the Lyman-alpha optical depth statistics in the proximity
regions of quasar spectra depend on the mass of the dark matter halos hosting
the quasars. This is owing to both the overdensity around the quasars and the
associated infall of gas toward them. For a fiducial quasar host halo mass of
(3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering in
the 2dF QSO Redshift Survey, we show that estimates of the ionizing background
(Gamma^bkg) from proximity effect measurements could be biased high by a factor
of ~2.5 at z=3 owing to neglecting these effects alone. The clustering of
galaxies and other active galactic nuclei around the proximity effect quasars
enhances the local background, but is not expected to skew measurements by more
than a few percent. Assuming the measurements of Gamma^bkg based on the mean
flux decrement in the Ly-alpha forest to be free of bias, we demonstrate how
the proximity effect analysis can be inverted to measure the mass of the dark
matter halos hosting quasars. In ideal conditions, such a measurement could be
made with a precision comparable to the best clustering constraints to date
from a modest sample of only about 100 spectra. We discuss observational
difficulties, including continuum flux estimation, quasar systematic redshift
determination, and quasar variability, which make accurate proximity effect
measurements challenging in practice. These are also likely to contribute to
the discrepancies between existing proximity effect and flux decrement
measurements of Gamma^bkg.Comment: 25 pages, including 14 figures, accepted by Ap
Coring the sedimentary expression of the early Toarcian Oceanic Anoxic Event: New stratigraphic records from the Tethys Ocean
The Toarcian Oceanic Anoxic Event (T-OAE) interval was cored at Colle di Sogno and Gajum in the Lombardy Basin (Southern Alps, northern Italy). The Sogno and Gajum cores recovered 26.83 and 31.18 stratigraphic metres, respectively, of pelagic sediments consisting of marly limestones, marlstone, marly claystone, and black shale. Drilling at both sites resulted in 100\u2009% recovery of unweathered material. The pelagic succession comprises a relatively expanded black shale interval of 4.98\u2009m in the Sogno core and 15.35\u2009m in the Gajum core, with lower and upper boundaries without evidence of hiatuses. The Sogno and Gajum cores can be considered reference sections for the pelagic lower Toarcian interval of the western Tethys and will provide high-resolution micropaleontological, inorganic and organic geochemical, isotopic multiproxy data. Integrated stratigraphy and cyclostratigraphy are predicted to result in estimates of durations and rates to model the ecosystem resilience to the extreme perturbations of the T-OAE and gain a better understanding of current global changes and help provide better projections of future scenarios
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