Low-latitude ionospheric turbulence observed by Aureol-3 satellite

Using PSD (Power Spectral Density) data on electron density and electric field variations observed on board Aureol-3 satellite at low-to-mid-latitude ionosphere we analyze a scale distribution of the ionospheric turbulence in a form <i>k<sup>-α</sup></i>, where <i>k</i> is the wave number and α is the spectral index. At first, high-resolution data in the near-equator region for several orbits have been processed. In this case the frequency range is from 6Hz to 100Hz (corresponding spatial scales from 80m to 1.3km), each power spectrum obeys a single power law fairly well, and the mean spectral indices are rather stable with α<sub><i>N</i></sub>=2.2±0.3 and α<sub><i>E</i></sub>=1.8±0.2, for the density and electric field, respectively. Then we produce a statistical study of 96 electric field bursts in the frequency range 10-100Hz from low-time resolution data (filter bank envelope). These bursts concentrate on the side of the Equatorial Anomaly crest (geomagnetic latitude 30-40°). Spectral indices of the bursts vary in the interval α<sub><i>E</i></sub>=2.0-2.5 but are fairly stable in seasons and local times. The electric field power of the burst has rather a large variability but has a relative increase in mean values for the summer and winter, as well as the daytime. The effect of major seismic activities toward the ionospheric turbulence is not conclusive either for the refractive index or for the electric field power. However, the mean value for the electric field power of bursts during seismic periods is larger than that for non seismic periods, and the statistical difference of the mean values is rather significant

Lattice thermal conductivity of disordered NiPd and NiPt alloys

Numerical calculations of lattice thermal conductivity are reported for the binary alloys NiPd and NiPt. The present work is a continuation of an earlier paper by us [PRB, 72, 214207 (2005)]which had developed a theoretical framework for the calculation of configuration-averaged lattice thermal conductivity and thermal diffusivity in disordered alloys. The formulation was based on the augmented space theorem combined with a scattering diagram technique. In this paper we shall show dependence of the lattice thermal conductivity on a series of variables like phonon frequency, temperature and alloy composition. The temperature dependence of $\kappa(T)$ and its realtion to the measured thermal conductivity is discussed. The concentration dependence of $\kappa$ appears to justify the notion of a minimum thermal conductivity as discussed by Kittel, Slack and others. We also study the frequency and composition dependence of the thermal diffusivity averaged over modes. A numerical estimate of this quantity gives an idea about the location of mobility edge and the fraction of states in the frequency spectrum which is delocalized.Comment: 23 pages, 18 figure

Influence of dynamic strain ageing on ductile fracture of a C-Mn steel

International audienceDuctile fracture of a C-Mn steel was characterised by tensile tests performed in a largetemperature range (from 20 to 350°C) on round notched specimens. The experimentalresults revealed a sharp drop of fracture strain around 250°C. This corresponds to thedomain of dynamic strain ageing (DSA) occurrence. The Portevin-Le Chatelier (PLC) effect,which is the most classical effect of DSA, was modelled on round notched specimens with amechanical behaviour model including the strain ageing effect. Local approach to fracturewas applied to predict the ductile fracture using the Rice and Tracey criterion. Thepredictions are rather close to the experimental results for the low and high temperatures. Inthe DSA domain, the approach predicts a decrease of the fracture strain but with a lowermagnitude than the experimental one. Therefore, DSA has an effect on mechanical behaviour but also on fracture mechanisms. Consequently, the fracture criterion has to bemodified in the presence of DSA

Contingent constraints of soil conservation innovations : Case of yam-based systems with herbaceous legumes in the Guinea-Sudan transition zone of Benin

One of the most serious problems of farming systems in West Africa is the excessive reduction of agricultural productivity related to the "slash and burn" and shifting cultivation systems. With the aim of designing more sustainable yam cropping systems, the agronomic research organization in Benin implemented alternative systems including herbaceous legumes (Aeschynomene histrix and Mucuna pruriens var utilis). This study examines with end-users in the framework of focus group and individual surveys in 306 farm households, constraints degree of severity of yam-based cropping systems and participatory solutions in order to improve policy transacting in rural areas for sustainable yam-production. The results showed significantly high constraint degree of severity for herbaceous legumes biomass incorporation (52% and 46%) for Mucuna and Aeschynomene respectively during individual survey and 82% during focus group for both legumes. Crop competitions, field access (with Mucuna), animal divagation, fertilizer cost, biomass burn, seed consumption, market and grains harvest (with Aeschynomene) were additional constraints as well as in low and relatively high population density zone. On the seed production plot, the biomass of Aeschynomene at senescence could be shaken within basins to collect easily the seeds and avoid lost. The mixture of Aeschynomene seeds with dry sand (3/4 sand - 1/4 seeds) solves the problem of planting small seeds. Before the legumes reach the physiological maturity, three-quarters of biomass could be manually incorporated into the soil before the dry season during ridging and the remaining biomass could be left on the surface as mulch in order to reduce the workload related to the biomass incorporation into the soil. The practice of fire wall and fire of reference around the plot is necessary to avoid the burn of the mulch in the dry period. Mucuna seeds valorisation occurred to generate additional incomes for smallholders' households. The animal nutrition with Mucuna grains deserves to be more investigated. The crop-livestock integration with these herbaceous should be an opportunity for yam production because of agro pastoral potential in the Guinea- Sudan transition zone of Benin. (Résumé d'auteur

Simulations of ionospheric turbulence produced by HF heating near the upper hybrid layer

Heating of the ionosphere by high-frequency (HF), ordinary (O) mode electromagnetic waves can excite magnetic field aligned density striations (FAS), associated with upper and lower hybrid turbulence and electron heating. We have used Vlasov simulations in one spatial and two velocity dimensions to study the induced turbulence in the presence of striations when the O-mode pump is mode converted to large amplitude upper hybrid oscillations trapped in a striation. Parametric processes give rise to upper and lower hybrid turbulence, as well as to large amplitude, short wavelength electron Bernstein waves. The latter excite stochastic electron heating when their amplitudes exceed a threshold for stochasticity, leading to a rapid increase of the electron temperature by several thousands of Kelvin. The results have relevance for high latitude heating experiments

Excitation of the ionospheric resonance cavity by neutral winds at middle latitudes

International audienceA new mechanism for the ionospheric Alfvén resonator (IAR) excitation at middle latitudes is considered. It is shown that the ionosphere wind system in this region is capable of sustaining the generation of geomagnetic perturbations that can be detected by ground magnetometers. The general IAR dispersion relation describing the linear coupling of the shear Alfvén and fast magnetosonic/compressional modes is obtained. The dependence of the IAR eigenfrequencies and damping rates on the perpendicular wave number and on the ground conductivity during the day- and nighttime conditions is analyzed both analytically and numerically. In order to demonstrate the IAR excitation by neutral winds the power spectra of the geomagnetic perturbation on the ground surface are calculated. Furthermore, it is found that Kolmogorov spectra of the ionospheric turbulent neutral winds and the IAR eigenfrequencies lie in the same frequency range that make it possible to enhance the IAR excitation. The relevance of the developed theoretical model to the ground-based observations is stressed

On Es-spread effects in the ionosphere connected to earthquakes

International audienceIn the present work, phenomena in the ionosphere are studied, which are connected with earthquakes (16 events) having a depth of less than 50 km and a magnitude M larger than 4. Analysed are night-time Es-spread effects using data of the vertical sounding station Petropavlovsk-Kamchatsky (f=53.0°, ?=158.7°) from May 2004 until August 2004 registered every 15 min. It is found that the maximum distance of the earthquake from the sounding station, where pre-seismic phenomena are yet observable, depends on the magnitude of the earthquake. Further it is shown that 1–2 days before the earthquakes, in the pre-midnight hours, the appearance of Es-spread increases. With a probability of more than 0.95, this increase of Es-spread observations before midnight is not casual

Variations of VLF/LF signals observed on the ground and satellite during a seismic activity in Japan region in May–June 2008

Signals of two Japanese transmitters (22.2 kHz and 40 kHz) recorded on the ground VLF/LF station in Petropavlovsk-Kamchatsky and on board the DEMETER French satellite have been analyzed during a seismic activity in Japan in May–June 2008. The period of analysis was from 18 April to 27 June. During this time two rather large earthquakes occurred in the north part of Honshu Island – 7 May (<i>M</i>=6.8) and 13 June (<i>M</i>=6.9). The ground and satellite data were processed by a method based on the difference between the real signal in nighttime and the model one. For ground observations a clear decrease in both signals has been found several days before the first earthquake. For the second earthquake anomalies were detected only in JJI signal. The epicenters of earthquakes were in reliable reception zone of 40 kHz signal on board the DEMETER. Signal enhancement above the seismic active region and significant signal intensity depletion in the magnetically conjugate area has been found for satellite observation before the first earthquake. Anomalies in satellite data coincide in time with those in the ground-based observation

Fractal Metrology for biogeosystems analysis

The solid-pore distribution pattern plays an important role in soil functioning being related with the main physical, chemical and biological multiscale and multitemporal processes of this complex system. In the present research, we studied the aggregation process as self-organizing and operating near a critical point. The structural pattern is extracted from the digital images of three soils (<i>Chernozem, Solonetz</i> and <i>"Chocolate" Clay</i>) and compared in terms of roughness of the gray-intensity distribution quantified by several measurement techniques. Special attention was paid to the uncertainty of each of them measured in terms of standard deviation. Some of the applied methods are known as classical in the fractal context (box-counting, rescaling-range and wavelets analyses, etc.) while the others have been recently developed by our Group. The combination of these techniques, coming from Fractal Geometry, Metrology, Informatics, Probability Theory and Statistics is termed in this paper <i>Fractal Metrology</i> (FM). We show the usefulness of FM for complex systems analysis through a case study of the soil's physical and chemical degradation applying the selected toolbox to describe and compare the structural attributes of three porous media with contrasting structure but similar clay mineralogy dominated by montmorillonites