Les champs magnétiques jouent un rôle important dans l'évolution stellaire, mais les propriétés magnétiques des étoiles massives et de masse intermédiaire sont mal connues. Seul une petite fraction (7%) des étoiles massives et de masse intermédiaire possèdent un champ magnétique et la force de leur champ dipolaire est supérieure à ~300 Gauss. La théorie pour expliquer l'origine de ces champs, la théorie des champs fossiles, n'explique pas pourquoi seulement une petite partie des étoiles chaudes ont un champ magnétique. Récemment, un champ magnétique ultra-faible (moins de 1 Gauss) a été découvert sur deux étoiles de masse intermédiaire (Vega et Sirius). Ce sont peut être les premières détections d'un nouveau type de champ magnétique faible. Deux familles d'étoiles magnétiques chaudes pourraient donc exister, avec des champs forts ou ultra-faibles, séparées par ce qu'on appelle le désert magnétique. Ma thèse consiste à analyser des données spectropolarimétriques prises avec des spectropolarimètres haute résolution, principalement avec Narval installé au télescope de 2 mètres à l'Observatoire du Pic du Midi. Une partie de ma thèse été dédiée à l'étude des champs magnétiques les plus faibles, parmi les champs forts. J'ai analysé les observations de l'étoiles O massive zeta Ori A. Peu d'étoiles O sont connues pour être magnétiques et zeta Ori A possède le plus faible champ magnétique. J'ai aussi participé à un programme observationnel pour déterminer la limite supérieure du désert magnétique grâce aux étoiles Ap/Bp. Le but de ces études est de tester la dépendance de la limite supérieure du désert magnétique par rapport à la rotation et à la masse. Une deuxième partie de ma thèse est consacrée à la recherche des champs ultra-faibles pour fournir des contraintes aux divers scenarios qui expliquent la dichotomie entre les champs forts et faibles et améliorer notre connaissance des propriétés ce type de champ magnétique. Je présente les résultats d'étude d'étoiles normales, UZ Lyn et Vega, ainsi que celles de plusieurs d'étoiles chimiquement particulières (Am et HgMn). Les études présentées dans ma thèse apportent une lumière nouvelle sur le magnétisme des étoiles chaudes et des contraintes pour la physique stellaire en général, en particulier pour l'évolution stellaire. / Magnetic fields are known to play a fundamental role in stellar evolution but the magnetic properties of massive and intermediate-mass stars are not well understood. Only a small (7%) fraction of massive and intermediate-mass stars are found to be magnetic and their dipolar magnetic field strength is above ~300 Gauss. The current paradigm, the fossil field theory, describes this magnetism as remnant of an early phase of the star-life, but leaves many basic questions unanswered, such as the small fraction of magnetic stars, and in practice provides no constraint to stellar evolution theory. Recently, an ultra weak magnetic field (less than 1 Gauss) has been discovered in two intermediate mass stars (Vega and Sirius). They may be the first detections of a new type of weak magnetic fields. Two families of magnetic stars may thus exist: with strong or ultra-weak fields, separated by the so-called magnetic desert. My PhD thesis consists in analyzing observational data taken with high-resolution spectropolarimeters, mainly with Narval installed on the 2-meter telescope at the Pic du Midi Observatory, to detect magnetic fields. One part of my thesis is dedicated to the study of the weakest end of strong magnetic fields. I analyzed the observations of a massive O star, zeta Ori A. Only a few O stars are known to be magnetic and zeta Ori A has the weakest field. I was also involved in a project to determine the upper limit of the magnetic desert thanks to observations of Ap/Bp stars. The goal of these studies is to test the dependence of the upper limit with rotation and mass. The other part of my thesis is dedicated to the search for ultra-weak fields in hot stars to provide constraints to the various scenarios that explain the strong vs weak field dichotomy and improve our understanding of the properties of this kind of weak field. I present the result of the studies of normal stars, UZ Lyn and Vega, and of several chemically peculiar (Am and HgMn) stars. The studies presented in my PhD thesis provide new clues about magnetism in hot stars and constraint for stellar physics in general, in particular for stellar evolution.

This thesis studies the acceleration of particles to super-thermal energies in explosive solar events as well as the magnetic changes in connectivity that may be responsible for changes in the morphology of quiescent filaments. Firstly a review of some of the observations of solar flare dynamics is given, as well as an introduction to the competing theories attempting to explain both particle acceleration and filament formation. An explanation of the numerical FORTRAN code that is used to calculate the trajectories of particle distribution functions in prescribed electromagnetic fields is given. Examples of known fields are used to test the accuracy of the code and the simple example of the well-known Litvinenko current sheet field is investigated. The results of charged particle orbit calculations in prescribed electric and magnetic fields motivated by magnetic reconnection models are then presented. The electromagnetic fields are chosen to resemble a current sheet with a localised reconnection region. The dependence of the model on the important physical parameters is considered. An introduction to the mathematical formulation of a collapsing magnetic trap is given. The same numerical code is used to calculate single electron orbits in this more complicated time dependent electromagnetic field. Consideration of important previous work is given before describing the best attempts to model the movement of flare loops in a realistic fashion. Finally the process of flux cancellation and filament formation is studied using a range of data including ground-based Hα and SoHO MDI magnetograms. It is found that the cancellation occurs at the ends of Hα sections of the filament and is accompanied by a noticeable increase in the Hα intensity and linkage of the sections. Measurements of the amount of flux cancelled at each site show it is in agreement with an estimate of the axial flux contained in the filament.

The problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.

O retalho miocutâneo transverso do abdome (TRAM) é um dos principais métodos utilizados na reconstrução mamária, sendo que este tipo de cirurgia apresenta grande incidência de necrose. O objetivo deste estudo foi verificar a eficácia do campo magnético estático (CME) e do ultrassom pulsado de baixa intensidade (UPBI) na viabilidade do retalho TRAM em ratos. Para tanto, quarenta e oito ratos machos foram distribuídos em quatro grupos homogêneos: G1 (Controle) - sem exposição ao campo magnético e ultrassom, G2- expostos ao campo magnético sul, G3 - expostos ao campo magnético Norte e G4 - estimulados com ultrassom pulsado de baixa intensidade. O campo magnético de um ímã consistirá de ferrite de bário (BaFe12O19). A viabilidade do retalho será avaliada pela porcentagem de área de necrose, eserá analisada por meio do software de processamento e análise de imagens Area®. A avaliação da circulação periférica do retalho foi obtida por meio de termografia infravermelha FLIR T450 (FLIR Systems - OregonUSA) antes, e durante três dias intervenções terapêuticas. Também foram avaliadas a quantidade de mastócitos e espessura de epiderme. A normalidade dos dados foi avaliada pelo teste de Shapiro-Wilk, sendo verificada distribuição normal. A porcentagem de área de necrose, espessura da epiderme e quantificação de mastócitos, foram avaliados pelo teste ANOVA one-way e pós-hoc de Tukey (p<0.05). A análise da temperatura cutânea obtida por termografia infravermelha foi avaliada pelo teste two-way de medidas repetidas seguido do teste de Bonferroni. O processamento dos dados foi efetuado pelo software SPSS (Chicago, IL, USA), versão 13.0, sendo fixado o nível crítico de 5% (p<0,05). Houve diferença significativa entre os grupos na porcentagem de área de necrose, na temperatura do retalho, bem como na quantidade de mastócitos. Pode-se concluir que as intervenções terapêuticas propostas podem interferir na viabilidade do retalho TRAM / The retail myocutaneos transverse abdominal (TRAM) is one of the main methods used in breast reconstruction is that this type of plastic surgery shows a higher incidence of necrosis. The objective of the static magnetic field (SMF) the TRAM flap in rats. Forty-eight male rats will be divided into three equal groups; G1 (control), without exposure to the magnetic field, G2- exposed to the magnetic field South and 3- exposed to the magnetic field north and group G4 - Stimulated with low intensity pulsed ultrasound. The magnetic field of a magneto f barium ferrite (BaFe12O19). The viability of the flap will be evaluated by the percentage of area of necrosis Esera analyzed by means of software processing and analyzing images Area®. The assessment of retail peripheral circulation was obtained by infrared thermography FLIR T450 (FLIR - OregonUSA) before and three days therapeutic interventions. They evaluated the amount of mast cells and epidermal thickness. Data normality was assessed by the Shapiro-Wilk test, and verified normal distribution. The percentage of necrotic area, epidermal thickness and quantification of mast cells were evaluated by one-way ANOVA and post-hoc Tukey (p <0.05). Analysis of the skin temperature obtained by infrared thermography test was evaluated by two-way repeated measures followed by Bonferroni test. Data processing was performed using SPSS software (Chicago, IL, USA), version 13.0, and set the critical level of 5% (p <0.05). There was a significant difference between the groups in the percentage of necrotic area in retail temperature and the amount of mast cells. In conclusion the therapeutic interventions proposed may interfere with the viability of the TRAM flap

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