Browsing
     by title


0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

or enter first few letters:   
OK
See detailLatitudinal and local time distribution of the O2 infrared nightglow and O density in the lower thermosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Saglam, Adem et al

Poster (2009, May)

Atomic oxygen is the major component in the Earth’s upper thermosphere. The O density reaches a maximum of about 3x1010 cm-3 near 100 km. The 1.27 µm emission of the IR Atmospheric bands generated by ... [more ▼]

Atomic oxygen is the major component in the Earth’s upper thermosphere. The O density reaches a maximum of about 3x1010 cm-3 near 100 km. The 1.27 µm emission of the IR Atmospheric bands generated by recombination of O atoms has been observed in the nightglow. On the night side, the O2 airglow peaks at ~95 km with a value of ~10 MR. <br />Atomic oxygen is present in the Martian atmos-phere but that the intensities values are below the cur-rent instrument detectability threshold. The Mars at-mosphere oxygen density is highly variable, depending on the altitude, temperature, latitude and longitude. <br />On Venus, atomic oxygen has been measured in situ only above 145 km on both the day and the night sides. Limb observations obtained with the Venus In-frared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express show that the O2 infrared nightglow peaks at ~97 km, with a mean intensity val-ue of about 1,3 MR [1, 2]. Yet, the distribution is largely inhomogeneous, with an enhanced region of ~3 MR statistically located near the midnight meridian at low latitude [3]. The oxygen density can be mapped using the O2 airglow and CO2 density vertical distribu-tions [4]. The O2 volume emission rates are obtained with an Abel inversion of the O2 limb profiles and CO2 vertical distributions are taken from the Venus International Reference Atmosphere (VIRA) model. The results show that the O density peak is located between 93 and 105 km (with a mean value of 104 km) and ranges from 2.8x1010 to 8.5x1011 cm-3 (with a mean value of 2.2x1011 cm-3). No correlations between the peak altitude and the latitude or the peak altitude and the local time are observed. However, the O density decreases and its variability increases while moving away from the antisolar point. [less ▲]

Detailed reference viewed: 27 (10 ULg)
Peer Reviewed
See detailThe latitudinal gradient of nitric oxide in the thermosphere
Cravens, T. E.; Gérard, Jean-Claude ULg; Stewart, A. I. et al

in Journal of Geophysical Research (1979), 84

Theoretical calculations of nitric oxide altitude profiles are made at five different latitudes by using neutral temperatures and composition primarily from the MSIS (mass spectrometer and incoherent ... [more ▼]

Theoretical calculations of nitric oxide altitude profiles are made at five different latitudes by using neutral temperatures and composition primarily from the MSIS (mass spectrometer and incoherent scatter) model. The nitric oxide calculated for an altitude of 105 km remains nearly constant with increasing latitude. Observations made by the ultraviolet nitric oxide instrument on the Atmosphere Explorer C satellite show that at low magnetic activity (Ap value of approximately 4), the NO density at 105 km agrees with the theory; however, at moderate levels of activity it increases with latitude. This discrepancy between the theoretical and observed latitudinal gradients of nitric oxide suggests the transport of NO from a high latitude source to lower latitudes. At 200 km the theoretical and observed latitudinal gradients are in reasonable agreement, an indication that the knowledge of the local composition and temperature is sufficient to model nitric oxide at this altitude. [less ▲]

Detailed reference viewed: 6 (0 ULg)
Peer Reviewed
See detailThe latitudinal gradient of the NO peak density
Fesen, C. G.; Rusch, D. W.; Gérard, Jean-Claude ULg

in Journal of Geophysical Research (1990), 95

Results are presented from SME observations of the latitudinal gradients of peak NO densities at about 110-km altitude during the solstice and equinox periods from 1982 through 1985. It is shown that the ... [more ▼]

Results are presented from SME observations of the latitudinal gradients of peak NO densities at about 110-km altitude during the solstice and equinox periods from 1982 through 1985. It is shown that the response of the peak NO densities to the declining level of solar activity varies with latitude, with the polar regions exhibiting low sensitivity and the low-latitude regions responding strongly. The SME data also revealed marked asymmetries in the latitudinal structure of the two hemispheres for each season and considerable day-to-day variations in the NO densities. The solar cycle minimum data for June were simulated using a two-dimensional model; results of sensitivity studies performed with varied quenching rate and eddy diffusion coefficient are presented. [less ▲]

Detailed reference viewed: 9 (0 ULg)
Full Text
Peer Reviewed
See detailLatitudinal patterns of magnitude and interannual variability in net ecosystem exchange regulated by biological and environmental variables
Yuan, Wenping; Luo, Yiqi; Richardson, Andrew D et al

in Global Change Biology (2009), 15(12), 2905-2920

Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the ... [more ▼]

Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO2 based on 163 site-years of eddy covariance data, from 39 northern-hemisphere research sites located at latitudes ranging from similar to 29 degrees N to similar to 64 degrees N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands. [less ▲]

Detailed reference viewed: 52 (2 ULg)
Full Text
Peer Reviewed
See detailLatitudinal structure of the Venus O2 infrared airglow: A signature of small-scale dynamical processes in the upper atmosphere
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Piccioni, Giuseppe et al

in Icarus (2014), 236

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the ... [more ▼]

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the latitudinal distribution of the O2(a1D) airglow limb profiles at 1.27 lm to characterize its distribution and variability. We show that the instantaneous structure of the emission is very different from the statistical global view of an enhanced emission near the equator, decreasing in brightness and slightly increasing in altitude toward the poles. The peak intensity of the limb profiles varies by a factor up to 50 between the brightest spots and the darkest regions. The bright airglow spots correspond to regions of enhanced downward flow of oxygen atoms originating from the dayside. Considerable variations in brightness and morphology are observed in the altitude–latitudinal distribution over a 24-h period. Analysis of the limb profiles indicates that secondary airglow peaks located at altitudes higher than the mean value of 96 km are observed on about 30% of the latitudinal cuts, but they are concentrated in narrow latitude areas extending over a few hundred kilometers. Most of them occur in transition regions between two altitude regimes in the 50 to 60 N region, possibly associated with the drop of the cloud top altitude observed equatorward of the ‘‘cold collar’’. We interpret these results as an indication that the strength of vertical transport in this mesosphere–thermosphere transition region is very variable both in location and time. This variability, also observed in nadir airglow images and wind measurements, is a key characteristic of the mesosphere–thermosphere transition region. It may be caused by fluctuations of the global day-to-night circulation generated by gravity waves. We show with a one dimensional model that local enhancements of eddy transport is a possibility. This variability is currently not accounted for by global circulation models that predict a single stable region of enhanced airglow in the vicinity of the antisolar point. [less ▲]

Detailed reference viewed: 16 (7 ULg)
Full Text
Peer Reviewed
See detailLattice Boltzmann 3D flow simulations on a metallic foam
Beugre, Djomice Antoine ULg; Calvo, Sébastien ULg; Dethier, Gérard ULg et al

in Journal of Computational & Applied Mathematics (2009)

Detailed reference viewed: 252 (32 ULg)
Full Text
Peer Reviewed
See detailLattice Boltzmann Simulation Code Optimization Based on Constant-time Circular Array Shifting
Dethier, Gérard ULg; de Marneffe, Pierre-Arnoul ULg; Marchot, Pierre ULg

in Procedia Computer Science (2011), 4

Lattice Boltzmann (LB) methods are a class of Computational Fluid Dynamics (CFD) methods for fluid flow simulation. LB simulation codes have high requirements regarding memory and computational power ... [more ▼]

Lattice Boltzmann (LB) methods are a class of Computational Fluid Dynamics (CFD) methods for fluid flow simulation. LB simulation codes have high requirements regarding memory and computational power: they may involve the update of several millions of floating point values thousands of times and therefore require several gigabytes of available memory and run for several days. Optimized implementations of LB methods minimize these requirements. An existing method based on a particular data layout and an associated implementation implying a constant time array shifting allows to reduce the execution time of LB simulations and almost minimize memory usage when compared to a naive implementation. In this paper, we show that this method can be further improved, both in memory usage and performances by slightly modifying the data layout and by using blocking in order to enhance data locality. [less ▲]

Detailed reference viewed: 150 (8 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics and anomalous softening in the YbFe4Sb12 skutterudite
Moechel, A.; Sergueev, I.; Wille, H.-C. et al

in Physical Review (2011), 84(18),

The lattice dynamics of the filled skutterudite YbFe4Sb12 was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at similar to 50 ... [more ▼]

The lattice dynamics of the filled skutterudite YbFe4Sb12 was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at similar to 50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin. [less ▲]

Detailed reference viewed: 13 (0 ULg)
Peer Reviewed
See detailLattice dynamics and ferroelectric instability of barium titanate
Ghosez, Philippe ULg; Gonze, X.; Michenaud, J. P.

in Ferroelectrics (1997), 194

Using first-principles calculations, we investigate the lattice dynamics of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we ... [more ▼]

Using first-principles calculations, we investigate the lattice dynamics of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we report results on the Born effective charges and the dielectric tensor and we emphasize the important reduction of these quantities in the ferroelectric phase, Then, the phonon frequencies at the Gamma point are calculated. We point out the similarity of theoretical eigenvectors in the cubic and rhombohedral phases. We examine the interaction of the vibration modes with the electric field and in particular the giant LO-TO splitting of the ferroelectric mode. Finally, separating the dipole-dipole interaction from the remaining short-range forces, we quantify the balance of forces leading to an unstable phonon in the cubic phase and we demonstrate its sensitivity to tiny effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase is produced by a reduction of the Born effective charges, while its stabilization under isotropic pressure is associated with a modification of the short-range forces. [less ▲]

Detailed reference viewed: 104 (3 ULg)
Full Text
See detailLattice dynamics and magnetic properties in selected functional iron compounds
Herlitschke, Marcus ULg

Doctoral thesis (2015)

The presented thesis reports about to the investigation of four different materials with respect to their magnetic properties and their lattice dynamics. The carbodiimide FeNCN, which was recently ... [more ▼]

The presented thesis reports about to the investigation of four different materials with respect to their magnetic properties and their lattice dynamics. The carbodiimide FeNCN, which was recently synthesized for the first time and can be considered as an non-oxidic analogue to FeO, is discussed in chapter 2. The NCN 2− group bridges two iron and exhibits a double negative charge, similar to the oxygen in FeO. This material exhibits an increased magnetic order temperature as compared to FeO. FeNCN was investigated with magnetometry, Mössbauer spectroscopy and nuclear inelastic scattering, revealing an unusual behaviour of the hyperfine parameters below the ordering temperature. This observation is discussed with respect to the crystal field and the d-orbital level population with temperature. The investigation of the magnetocaloric material Mn5−x FexSi3 is reported in chapter 3 with focus on the interaction of magnetism and lattice dynamics. Among the Mn5−xFe x Si3 compounds, MnFe4 Si3 is the most interesting one due to the possible application as a room temperature refrigeration material. The investigation reveals that a direct measurement of the influence of the magnetocaloric effect on the density of iron phonon states is not possible, however, the elasticity was observed to be strongly influenced by this effect. The studies on the spin orientation in nanospheres and a NANOPERM alloy are discussed in chap ter 4 and chapter 5. Two approaches were followed in order to investigated the spin orientation, nuclear forward scattering and nuclear resonant small-angle X-ray scattering. In the first part, maghemite nanospheres are analysed with in-field nuclear forward scattering, revealing that the spins orient only partially along the direction of the applied magnetic field. A modeled magnetization calculated from the observed degree of spin orientation matches excellently the macroscopic magnetometry data and literature values. Though revealing the amount of oriented spins, nuclear forward scattering is not sensitive to their position in the nanoparticles. Thus, the nuclear forward scattering was combined with small-angle X-ray scattering, leading to small-angle patterns in dependence of the local spin orientation in the aforementioned samples. [less ▲]

Detailed reference viewed: 34 (6 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics and structure of GeTe, SnTe and PbTe
Pereira, Paula Bauer; Sergueev, Ilya; Gorsse, Stephane et al

in PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS (2013), 250(7), 1300-1307

The lattice dynamics in the IV-VI compounds GeTe, SnTe and PbTe were studied by Te-125 and Sn-119 nuclear inelastic scattering and the obtained partial density of phonon states were compared with ... [more ▼]

The lattice dynamics in the IV-VI compounds GeTe, SnTe and PbTe were studied by Te-125 and Sn-119 nuclear inelastic scattering and the obtained partial density of phonon states were compared with published theoretical calculations. The phase purity and structure were characterized by high energy X-ray diffraction. The effect of the atomic arrangement, rhombohedral for GeTe and cubic for SnTe and PbTe, is visible in the density of phonon states. Vibrational properties are found to be in good agreement with available calculated data and the softer character of the NaCl-type structures in comparison with the rhombohedral GeTe is confirmed. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [less ▲]

Detailed reference viewed: 951 (0 ULg)
Full Text
See detailLattice dynamics in antimony and tellurium based phase-change materials
Simon, Ronnie Ernst ULg

Doctoral thesis (2015)

Detailed reference viewed: 36 (4 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics in Bi2Te3 and Sb2Te3: Te and Sb density of phonon states
Bessas, D.; Sergueev, I.; Wille, H.-C. et al

in PHYSICAL REVIEW B (2012), 86(22), 224301-9

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements ... [more ▼]

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi2Te3. A prominent peak in the Te specific density of phonon states at 13 meV, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a similar to 20% force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at 20 K in Bi2Te3, 1.75(1) km/s, compared to Sb2Te3, 1.85(4) km/s, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at 295 K, 2.4 Wm(-1)K(-1) for Sb2Te3 and 1.6 Wm(-1)K(-1) for Bi2Te3, cannot be explained by anharmonicity alone given the rather modest Gruneisen parameters, 1.7(1) for Sb2Te3 and 1.5(1) for Bi2Te3, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy. [less ▲]

Detailed reference viewed: 37 (0 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics in intermetallic Mg2Ge and Mg2Si
Bessas, D.; Simon, R. E.; Friese, K. et al

in Journal of Physics : Condensed Matter (2014), 26(48),

The lattice dynamics of polycrystalline Mg2Ge and Mg2Si are compared using both microscopic and macroscopic measurements as well as theoretical calculations. The volume thermal expansion coefficient ... [more ▼]

The lattice dynamics of polycrystalline Mg2Ge and Mg2Si are compared using both microscopic and macroscopic measurements as well as theoretical calculations. The volume thermal expansion coefficient between 200 and 300K was found to be 4.37(5)•10-5K-1 in Mg2Ge, compared to 3.69(5)•10-5K-1 in Mg2Si. Inelastic neutron scattering measurements yield densities of phonon states which are in line with theoretical calculations. The microscopic data were corroborated with macroscopic calorimetry measurements and provide quantified values for anharmonicity. The estimated macroscopic Grneisen parameter is, γMg 2Si=1.17(5) and γMg 2Ge=1.46(5) at 295K, in excellent agreement with Raman scattering data. Although the element specific mean force constants are practically the same, in Mg2Ge and Mg2Si, a mass homology relation alone cannot reproduce the difference in the partial densities of vibrational states in these compounds and differences in elemental bonding should be taken into account. © 2014 IOP Publishing Ltd. [less ▲]

Detailed reference viewed: 8 (1 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics in the FeSb3 skutterudite
Moechel, A.; Sergueev, I.; Long, Gary J. et al

in Physical Review (2011), 84(6),

Thin films of FeSb3 were characterized by electronic transport magnetometry, x-ray diffraction, Fe-57 and Sb-121 nuclear inelastic scattering, and Fe-57 Mossbauer spectroscopy. Resistivity and ... [more ▼]

Thin films of FeSb3 were characterized by electronic transport magnetometry, x-ray diffraction, Fe-57 and Sb-121 nuclear inelastic scattering, and Fe-57 Mossbauer spectroscopy. Resistivity and magnetometry measurements reveal semiconducting behavior with a 16.3(4) meV band gap and an effective paramagnetic moment of 0.57(6) mu(B) respectively. A systematic comparison of the lattice dynamics with CoSb3 and EuFe4Sb12 reveals that the Fe4Sb12 framework is softer than the Co4Sb12 framework, and that the observed softening and the associated lowering of the lattice thermal conductivity in the RFe4Sb12 filled skutterudites are not only related to the filler but also to the Fe4Sb12 framework. [less ▲]

Detailed reference viewed: 29 (3 ULg)
Full Text
Peer Reviewed
See detailLattice dynamics in the thermoelectric Zintl compound Yb14MnSb11
Möchel, A.; Sergueev, I.; Wille, H.-C. et al

in Physical Review (2011), 84(18),

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering ... [more ▼]

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb14MnSb11 contrasts with the low energy of all phonons (<25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu14MnSb11 compound and of the thermoelectric Zintl phase Zn4Sb3 in order to compare with related thermodynamic properties of Yb14MnSb11 and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases. [less ▲]

Detailed reference viewed: 22 (0 ULg)
Full Text
See detailLattice dynamics in thermoelectric Zintl phases
Möchel, Anne ULg

Doctoral thesis (2011)

A brief introduction to the thermoelectric effects, the studied materials, and the experimen- tal methods is given in the first chapter. The introduction is not intended to be exhaustive, but only to ... [more ▼]

A brief introduction to the thermoelectric effects, the studied materials, and the experimen- tal methods is given in the first chapter. The introduction is not intended to be exhaustive, but only to summarize important basic information for the reader. The introduction is followed by four chapters dedicated to detailed experimental studies of the lattice dynamics in selected thermoelectric Zintl phases. First, the lattice dynamics in the unfilled and filled skutterudites FeSb3 , CoSb3 , and YbFe4 Sb12 were studied by nuclear inelastic scattering, inelastic neutron scattering, and several macroscopic methods. These studies reveal that FeSb3 exhibits softer Sb bonds than CoSb3 , that the density of phonon states is shifted towards lower energies and the velocity of sound is lower in FeSb3 as compared to CoSb3 . It appears thus that the soft [Fe4 Sb12 ] framework dynamics might play an important role in the thermoelectric properties of filled skutterudites. The observed anomalous temperature dependence of the elastic constants and the rearrangement of the spectral weight of the Yb phonon states in YbFe4 Sb12 can be explained by a change of the Yb valence state with the temperature. Second, the lattice dynamics in the Sr8 Ga16 Ge30 clathrate was investigated by inelastic neutron scattering measurements on a single crystal. We found that several mechanisms contribute to the low thermal conductivity in this system and that the reduction of the heat capacity contribution to the thermal conductivity plays a significant role to the low thermal conductivity, besides the reduction in the phonon lifetime and the phonon group velocity that is related to the guest atom. Third, the lattice dynamics of the Zintl phase Yb14 MnSb11 was studied by inelastic neu- tron scattering and nuclear inelastic scattering measurements. All phonon modes of these complex crystal systems are in a narrow energy range below ∼25 meV and the Debye tem- perature, the velocity of sound and the mean force constants are small compared to those of other thermoelectric materials such as Zn4 Sb3 . By comparing the lattice dynamics in Yb14 MnSb11 and Zn4 Sb3 different mechanisms which lead to the low thermal conductivity in Zintl phases have been identified. Between 300 and 1200 K no softening of the velocity of sound in Yb14 MnSb11 was observed by temperature dependent inelastic neutron scattering measurements, which is in line with its large melting temperature. [less ▲]

Detailed reference viewed: 145 (7 ULg)