Manifestations de la sécheresse en Afrique de l'Ouest non sahélienne : cas de la Côte d'Ivoire, du Togo et du Bénin

La sécheresse qui sévit depuis une vingtaine d'années dans les régions sahéliennes d'Afrique de l'Ouest semble avoir des manifestations également plus au sud dans les pays riverains du golfe de Guinée. Une double analyse, ponctuelle et spatialisée, concernant les précipitations annuelles de la Côte d'Ivoire, du Togo et du Bénin permet de mettre ce fait en évidence. Les séries chronologiques d'indices pluviométriques confirment la chute brutale de la pluviométrie à la fin des années 60. La représentation cartographique des résultats montre le net glissement des courbes isohyètes vers le sud et permet de prendre en compte la dimension régionale du phénomène. (Résumé d'auteur

Variational inference of fractional Brownian motion with linear computational complexity

We introduce a simulation-based, amortised Bayesian inference scheme to infer the parameters of random walks. Our approach learns the posterior distribution of the walks' parameters with a likelihood-free method. In the first step a graph neural network is trained on simulated data to learn optimized low-dimensional summary statistics of the random walk. In the second step an invertible neural network generates the posterior distribution of the parameters from the learnt summary statistics using variational inference. We apply our method to infer the parameters of the fractional Brownian motion model from single trajectories. The computational complexity of the amortized inference procedure scales linearly with trajectory length, and its precision scales similarly to the Cram{\'e}r-Rao bound over a wide range of lengths. The approach is robust to positional noise, and generalizes well to trajectories longer than those seen during training. Finally, we adapt this scheme to show that a finite decorrelation time in the environment can furthermore be inferred from individual trajectories

Identifying neural substrates of competitive interactions and sequence transitions during mechanosensory responses in Drosophila.

Nervous systems have the ability to select appropriate actions and action sequences in response to sensory cues. The circuit mechanisms by which nervous systems achieve choice, stability and transitions between behaviors are still incompletely understood. To identify neurons and brain areas involved in controlling these processes, we combined a large-scale neuronal inactivation screen with automated action detection in response to a mechanosensory cue in Drosophila larva. We analyzed behaviors from 2.9x105 larvae and identified 66 candidate lines for mechanosensory responses out of which 25 for competitive interactions between actions. We further characterize in detail the neurons in these lines and analyzed their connectivity using electron microscopy. We found the neurons in the mechanosensory network are located in different regions of the nervous system consistent with a distributed model of sensorimotor decision-making. These findings provide the basis for understanding how selection and transition between behaviors are controlled by the nervous system

Accompagner les élèves ? : appropriation de la réforme, ré-élaboration dans le travail réel

La mise en œuvre des réformes n’est souvent regardée que comme application, sous l’angle de la conformité, sans considérer vraiment les richesses d’inventions, de créations ordinaires. Contrairement à ce qui se dit parfois, les professionnels ne sont pas résistants au changement, ils veulent être acteurs et auteurs de leurs contributions à ces améliorations, en puisant dans leurs expériences. Dès qu’ils en ont la possibilité, ils explorent des zones laissées dans l’ombre dans la mesure où ils estiment que cela pourrait constituer des opportunités pour expérimenter de possibles améliorations. Il y a alors des traductions, des expérimentations, des expériences redécouvertes, des ajustements progressifs et des débats sur les qualités des produits visés et réalisés dans le travail.C’est précisément ce que les chercheurs ont rencontré dans le travail ordinaire des professionnels avec qui ils ont partagé cette intervention-recherche d’une durée de trois ans.Après une présentation de l’intervention-recherche et de son déroulement dans quatre établissements et avec un groupe de directeurs-adjoints, le rapport de recherche focalise sur trois domaines : Les dispositifs d’AP et leur élaboration ; Le potentiel d’apprentissages des dispositifs d’AP pour les élèves ; Le travail collectif dans la mise en œuvre de l’accompagnement personnalisé (à l’orientation). La synthèse permet de mettre en lumière les processus d’appropriation de la réforme observés et leurs effets sur la dynamique des rapports entre activité des acteurs et institution. En conclusion, les auteurs ouvrent quelques pistes de réflexion à propos des contenus, modalités et conditions de la mise en œuvre et de l’appropriation d’une réforme éducative par les acteurs

Experimental evidence of Alfv\'en wave propagation in a Gallium alloy

Experiments with a liquid metal alloy, galinstan, are reported and show clear evidence of Alfv\'en wave propagation as well as resonance of Alfv\'en modes. Galinstan is liquid at room temperature, and although its electrical conductivity is not as large as that of liquid sodium or NaK, it has still been possible to study Alfv\'en waves, thanks to the use of intense magnetic fi elds, up to 13 teslas. The maximal values of Lundquist number, around 60, are similar to that of the reference experimental study by Jameson [1]. The generation mechanism for Alfv\'en waves and their refl ection is studied carefully. Numerical simulations have been performed and have been able to reproduce the experimental results despite the fact that the simulated magnetic Prandtl number was much larger than that of galinstan. An originality of the present study is that a poloidal disturbance (magnetic and velocity fields) is generated, allowing us to track its propagation from outside the conducting domain, hence without interfering.Comment: 19 pages; Physics of Fluids (2011)

Surface‐enhanced Raman scattering and photothermal effects on optoplasmonic nanofibers

When decorated with plasmonic nanoparticles, pulled optical nanofibers are compatible with plasmonic techniques enabling the ability to probe microenvironments with high spatial and temporal resolution. Although the nanofibers exhibit excellent compatibility for biological samples including cells and tissues, the underlying interactions between the dielectric fiber, plasmonic nanoparticles, and the incident light have been minimally explored. It is shown that the complex coupling of optical and plasmonic properties within the nanofiber strongly influences both the surface‐enhanced Raman scattering (SERS) and photothermal capabilities. Through a combination of experimental results and simulated electric field distributions and spectra it is demonstrated that, although the nanofibers may be homogeneously decorated with gold nanoparticles, the optical effects spatially differ. Specifically, the SERS performance varies periodically based on the diameter of the nanofiber, which is associated with ring resonator modes, while the photothermal effects are more homogeneous over the same diameters, highlighting differences in optoplasmonic properties at this length scale. Through understanding these effects, it may become possible to control temperatures and SERS properties to evaluate processes with micrometric spatial resolution, such as the analytes secreted during temperature‐induced death of single cells

Predicting leakage of the VERCORS mock-up and concrete containment buildings - a digital twin approach

EDF operates a nuclear power generation fleet made up of 56 reactors. This fleet contains 24 reactors designed as double-walled concrete containment building. The inner concrete containment vessel has no metallic liner and is a prestressed reinforced concrete building. The inner concrete containment vessel is designed to withstand a severe accident, in terms of mechanical and sealing behaviour. The tightness of the containment is tested every 10 years, by carrying out a pressurization test and by measuring the leak rate. The leak rate is required to be below a regulatory threshold to continue operation of the concrete containment building for the next ten years. Ageing of concrete due to drying, creep and shrinkage leads to increase prestress loss and then leak rate with time. For some containment buildings, the leak rate gets closer to the regulatory threshold with time, so important coating programs are planned to mitigate and limit the leak rate under the regulatory threshold. Therefore, it is very important for EDF to have a concrete containment building leak rate prediction tool. To address this issue, an important research program around a 1/3 scale concrete containment building mock-up called "VERCORS" have been launched at EDF. The mock-up is heavily instrumented, and its materials (concrete, prestressing cables) have been widely characterized and studied. An important numerical effort has also been made to implement structural computations of the mock-up and to capitalize these computations as well as their post-processing (so as to compare automatically with the monitoring data) in what can be called a digital twin of the mock-up. This digital twin is now used to predict the leakage of VERCORS mock-up before yearly pressure test, and also to optimize the repair programs on the real containments