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The Page-Rényi parking process
No full textInternational audienceIn the Page parking (or packing) model on a discrete interval (also known as the discrete Rényi packing problem or the unfriendly seating problem), cars of length two successively park uniformly at random on pairs of adjacent places, until only isolated places remain. We give a probabilistic proof of the (known) fact that the proportion of the interval covered by cars goes to 1-exp(-2) , when the length of the interval goes to infinity. We obtain some new consequences, and also study a version of this process defined on the infinite line
Estimation of connectivity in marine biological networks: graph theory versus metapopulation models. An application to the Gulf of Lion
No full textInternational audienceDifferent methods have been proposed for assessing important emerging properties for species conservation induced by biological networks connectivity. Graph theory has recently been used to investigate species persistence conditions from different measures of connectivity in this kind of networks. In this study, for the first time,we compare a set of metrics defined in graph theory to a metapopulation modelling approach. These approaches are confronted to evaluate the persistence of soft bottom polycheates populations in the Gulf of Lion,relying on the same connectivity matrices derived from larval dispersal simulations. Various classical graph analysis methods are tested while new descriptors are derived and evaluated. Most outstanding input of this work is the introduction of a novel metric for measuring the distance between nodes in a graph in order to correct an overlooked difficulty in previous literature, when dealing with connectivity matrices containing larval transfer probabilities. This metric ensures a physically meaningful interpretation of shortest paths and consequently betweenness. The comparison with metapopulation model enables to single out the interpretation for species persistence of the graph descriptors.Graph theory complements the results of the metapopulation model by adding an exhaustive analysis of the spatial information. In particular,modularity and bridging centrality are shown to quantify clusters of strongly interconnected nodes (communities) and to distinguish the rescuing sites and the sink sites. The sites bringing species' regional persistence seem better indicated by bridging centrality and shortest cycles lengths
From Geometric Semantics to Asynchronous Computability
No full textInternational audienceWe show that the protocol complex formalization of fault-tolerant protocols can be directly derived from a suitable semantics of the underlying synchronization and communication primitives, based on a geometrization of the state space. By constructing a one-to-one relationship between simplices of the protocol complex and (di)homotopy classes of (di)paths in the latter semantics, we describe a connection between these two geometric approaches to distributed computing: protocol complexes and directed algebraic topology. This is exemplified on atomic snapshot, iterated snapshot and layered immediate snapshot protocols, where a well-known combinatorial structure, interval orders, plays a key role. We believe that this correspondence between models will extend to proving impossibility results for much more intricate fault-tolerant distributed architectures
How much does weather control fire size and intensity in the Mediterranean region?
No full textInternational audienceThis study investigates the synoptic conditions favorable to wildfires in the Mediterranean region, in terms of fire intensity and burnt area. As reported in the literature, Mediterranean large wildfires are associated with a blocking situation. However, this study shows the existence of two types of wildfires controlled by the blocking high intensity: (1) fast build-up of a weak blocking produces intense wildfires associated with strong winds which allow propagation over long distances; (2) longer build-up of strong blocking situation produces less intense wildfires associated with weaker winds which also propagate over long distances. Another major step forward of this study in the understanding of the drivers of those wildfires is the evidence of a perfect match between the period of wildfire activity and the persistence of the favorable synoptic conditions: the wildfire activity starts at the onset of the blocking situation and ends with the transition to a less favorable synoptic weather pattern. Such strong control of the wildfire activity by the concomitant weather is a very promising result regarding fire risk management, especially considering the accidental nature of the Mediterranean wildfires
Clustering dans les réseaux par maximisation de modularité avec des contraintes de cohésion}
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Membrane protrusion powers clathrin-independent endocytosis of interleukin-2 receptor.
No full textInternational audienceEndocytosis controls many functions including nutrient uptake, cell division, migration and signal transduction. A clathrin- and caveolin-independent endocytosis pathway is used by important physiological cargos, including interleukin-2 receptors (IL-2R). However, this process lacks morphological and dynamic data. Our electron microscopy (EM) and tomography studies reveal that IL-2R-pits and vesicles are initiated at the base of protrusions. We identify the WAVE complex as a specific endocytic actor. The WAVE complex interacts with IL-2R, via a WAVE-interacting receptor sequence (WIRS) present in the receptor polypeptide, and allows for receptor clustering close to membrane protrusions. In addition, using total internal reflection fluorescent microscopy (TIRF) and automated analysis we demonstrate that two timely distinct bursts of actin polymerization are required during IL-2R uptake, promoted first by the WAVE complex and then by N-WASP. Finally, our data reveal that dynamin acts as a transition controller for the recruitment of Arp2/3 activators required for IL-2R endocytosis. Altogether, our work identifies the spatio-temporal specific role of factors initiating clathrin-independent endocytosis by a unique mechanism that does not depend on the deformation of a flat membrane, but rather on that of membrane protrusions
Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation
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Wave drag on a submerged sphere
No full textInternational audienceWe measure the wave drag acting on fully submerged spheres as a function of their depth and velocity, with an apparatus that measures only the component of the drag due to the proximity of the free surface. We observe that close to the surface the wave drag is of the order of the hydrodynamic drag. In our range of study, the measured force is more than one order smaller than predictions based on linear response. In order to investigate this discrepancy, we measure the amplitude of the waves at the origin of the wave drag, comparing the measurement with a theoretical model. The model captures the measurements at "large depth" but the wave's amplitude saturates at "small depth," an effect that partially accounts for the difference between the predicted and measured wave drag. (C) 2015 AIP Publishing LLC
Etude des couches frontières dans les plasmas : Structure et stabilité de la magnétopause terrestre
No full textThe terrestrial magnetopause is the boundary between the solar wind (compressed by a shock) and the terrestrial magnetosphere. This kind of thin and nearly impenetrable boundary naturally forms each time two magnetized plasmas are pushed one toward another. It happens here, like for several astrophysical situations, in a collisionless medium. For these reasons, the terrestrial magnetopause, accessible experimentally with a lot of satellite missions, is representative of a very general type of interfaces. Key phenomena like plasma transport across the boundary, heating and acceleration of charged particles or magnetic reconnection, take place at these interfaces. Therefore, studying and deeply understanding such kind of boundary is critical to understand the fundamental plasma physics.The terrestrial magnetopause is the boundary between two plasmas of different densities and temperatures. The magnetic fields of the magnetosphere and the solar wind have also different directions and intensities. The transition observed at the magnetopause therefore concerns matter, with two interpenetrating plasmas, and fields. How do these different kinds of variations combine and what structure does it give to the boundary? These arethe questions we study in this work. The simplest case, when the boundary can be considered locally as a plane and is stationary, will be the basis of the study, but we will also show how a boundary shaken by instabilities and magnetic reconnection can deviate from these simple models.In the first part of the thesis, we show an experimental study of the magnetopause using the data from the European Cluster mission. We show how to combine magnetic and ion data to obtain a characterization of the normal direction to the boundary and a coordinate along this normal, and validatethis new tool. Then, we show that when the normal magnetic field is nonzero, the boundary can be a succession of small layers bearing separately the rotational and compressional variations. We give clues on the good way to study these in detail.In the second part of the thesis, we develop a theoretical model of the structure as a 1D-stationay equilibrium of a current layer like the magnetopause.This equilibrium is a kinetical one, that means it is valid for the distribution function, and not only its first moments like density, fluid velocity, and pressure. This is necessary in a collisionless medium as soon as the characteristic scale of the particle motion, particularly the Larmor radius, is not negligible with respect to the thickness of the layer. Such kinds of equilibria are necessary to initialize the numerical simulations that are used to study the magnetopauseand the instabilities that can happen at the boundary like the tearing instability (which implies reconnection). Finally, we present a new tool for building Fourier spectra and phases for space plasmas turbulence studies.La magnétopause terrestre est la frontière entre le vent solaire (comprimé par une onde de choc) et la magnétosphère de la Terre. Ce type de frontière fine et quasiment étanche se forme naturellement chaque fois que deux plasmas magnétisés se trouvent projetés l’un vers l’autre. Il se produit, dans ce cas comme dans beaucoup d’autres situations astrophysiques, dans un milieu sans collisions. À ce titre, la magnétopause terrestre, qui est accessible à de nombreuses mesures satellitaires in-situ, est exemplaire d’un type très général d’interfaces. Des phénomènes clefs, comme le transport du plasma à travers la frontière, le chauffage et l’accélération des particules chargées ou la reconnexion magnétique, ont lieu à ces interfaces. L’étude et la compréhension détaillées de telles frontières est donc critique pour comprendre la physique fondamentale des plasmas.La magnétopause sépare deux régions de densités et de températures différentes. De plus, le champ magnétique de la magnétosphère et celui porté par le vent solaire n’ont ni la même direction ni la même intensité. On observe donc à la magnétopause une transition qui concerne à la fois la matière, avec deux plasmas qui s’interpénètrent, et les champs. Comment se combinent ces différents types de variations et quelle structure cela donne-t-il à la frontière ? Cesont les questions que nous étudions dans ce manuscrit. Le cas le plus simple, où la frontière peut être considérée comme localement plane et stationnaire, sera la base de l’étude, mais on montrera aussi dans quelle mesure une frontière agitée par des instabilités et la reconnexion magnétique peut s’écarter de ces modèles les plus simples.Dans la première partie de la thèse, nous effectuons une étude expérimentale de la magnétopause, en utilisant les données de la mission européenne Cluster. Nous montrons comment on peut combiner données magnétiques et ioniques pour obtenir une caractérisation de la normale à la frontière et une coordonnée le long de cette normale, et nous validons ce nouvel outil. Ensuite nous montrons que, lorsque le champ magnétique normal est non nul, la frontière peut former une succession de sous-couches portant séparément les variations de type rotationnelles et compressionnelles. Nous donnons des pistes sur la manière appropriée de les étudier en détail.Dans la seconde partie de la thèse, nous développons un modèle théorique de structure d’équilibre 1-D stationnaire d’une couche de courant de type magnétopause. Cet équilibre est de type "cinétique", ce qui signifie qu’il est valable au niveau de la fonction de distribution et pas seulement au niveau de ses premiers moments, densité, vitesse fluide et pression. Ceci est nécessaire dans un milieu sans collisions dès que les échelles caractéristiques du mouvement des particules, en particulier le rayon de Larmor, ne sont pas tout à fait négligeables vis-à-vis de l’épaisseur de la couche. Un tel équilibre est nécessaire pour initialiser les simulations numériques qui permettent de décrire de manière réaliste une frontière comme la magnétopause et d’étudier le développement d’instabilités telles que l’instabilité de déchirement (qui implique un phénomène de reconnexion).Enfin, nous présentons un nouvel outil pour la construction des spectres et des phases de Fourier dans le cadre des études de turbulence dans les plasmas spatiaux