Le mot du directeur de la publication

Avec la parution de ce premier numéro de la revue du CRREF, nous assistons à la manifestation concrète d’une évolution qui trouve sa source dans toute l’énergie déployée depuis quelques années par les uns et les autres pour prouver qu’une activité de recherche était possible au sein de l’IUFM de Guadeloupe. L’absence dans notre île et plus largement au sein de l’Université des Antilles et de la Guyane, des laboratoires de recherche en sciences sociales ou en sciences de l’éducation, ne consti..

Etude des couches frontières dans les plasmas : Structure et stabilité de la magnétopause terrestre

The 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

Analyzing institutional changes in community-based management : a case study of a participatory guarantee system for organic labeling in Brazil

The literature on collective action has poorly addressed processes of incremental institutional changes within organizations. This paper helps fill this gap by shedding light on how the community-based management of an organic label has changed following its formal recognition as a ‘participatory guarantee system’ (PGS). Ostrom and Basurto’s (2011) analytical tool is useful to describe the changes in collective rules to address collective action problems that take place in standard-setting and labeling activities. Using original data collected from the Ecovida Agroecology Network – the oldest and largest PGS in Brazil –, we study the multi-scale changes in governance rules from the 1970s until today. We pay particular attention to the links between the institutional consolidation of PGS, its recognition by public authorities, and its adaptation to legal rules. We detail analytical operators indicating whether the introduction of the PGS rules at the national level was bottom-up or top-down. Our findings emphasize how local communities have been able to both adapt their governance system in response to the official arrangements concerning organic regulations, and influence national public arenas where these arrangements are defined. This form of co-management has been accompanied by a significant increase in the number of PGS-certified farmers

BV technique for investigating 1-D interfaces

To investigate the internal structure of the magnetopause with spacecraft data, it is crucial to be able to determine its normal direction and to convert the measured time series into spatial profiles. We propose here a new single-spacecraft method, called the BV method, to reach these two objectives. Its name indicates that the method uses a combination of the magnetic field (B) and velocity (V) data. The method is tested on simulation and Cluster data, and a short overview of the possible products is given. We discuss its assumptions and show that it can bring a valuable improvement with respect to previous methods.Comment: submitted to JG

Wireless Communication in Dynamic Interference

International audienceFast varying active transmitter sets are a key feature of wireless communication networks with very short transmissions arising in machine-to-machine communications. A consequence is that the interference is dynamic, leading to non-Gaussian statistics. In this paper, we study the behavior of large-scale communication networks in the presence of isotropic α-stable interference, which forms a model for dynamic interference. We first characterize the achievable rate of each link by considering a non-Gaussian input distribution, which is shown to outperform a Gaussian input. Moreover, we analyze the area spectral efficiency, which is the total rate per square meter. Our analysis suggests that analogously to the common model of slowly varying active transmitter sets, dense networks maximize the area spectral efficiency

Choroidal melanoma recurrence after episcleral brachytherapy and transpupillary thermotherapy

ABSTRACT A 68-year-old man diagnosed with choroidal melanoma (CM) in the right eye underwent treatment with episcleral brachytherapy (I125) and transpupillary thermotherapy. Ultrasound, computed tomography, and magnetic resonance imaging were performed and revealed ocular recurrence of CM. Treatment with extended enucleation was performed. Macroscopic and microscopic examinations revealed extraocular extension and malignant cells, respectively. Immunohistochemistry demonstrated tumoral Melan-A and HMB-45 expression. No cytogenic abnormalities were detected with fluorescence in situhybridization of tumor cells using probes against chromosomes 3q27 and 8q24. The patient underwent adjuvant external beam radiotherapy for treatment of residual tumor tissue. This case represents the first reported case of recurrent CM with no cytogenetic abnormalities and the absence of metastatic disease, despite a number of the poorest prognostic factors

Exact Vlasov-Maxwell equilibria for asymmetric current sheets

The NASA Magnetospheric Multiscale mission has made in situ diffusion region and kinetic-scale resolution measurements of asymmetric magnetic reconnection for the first time, in the Earth's magnetopause. The principal theoretical tool currently used to model collisionless asymmetric reconnection is particle-in-cell simulations. Many particle-in-cell simulations of asymmetric collisionless reconnection start from an asymmetric Harris-type magnetic field but with distribution functions that are not exact equilibrium solutions of the Vlasov equation. We present new and exact equilibrium solutions of the Vlasov-Maxwell system that are self-consistent with one-dimensional asymmetric current sheets, with an asymmetric Harris-type magnetic field profile, plus a constant nonzero guide field. The distribution functions can be represented as a combination of four shifted Maxwellian distribution functions. This equilibrium describes a magnetic field configuration with more freedom than the previously known exact solution and has different bulk flow properties