Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2016-02-03T11:57:50Z No. of bitstreams: 0 / Made available in DSpace on 2016-02-03T11:57:50Z (GMT). No. of bitstreams: 0 / Neste trabalho foi realizado um estudo sistemático, das interações hiperfinas (campo hiperfino magnético e campo hiperfino elétrico), nos compostos do tipo RMn2Si2 e RMn2Ge2 (onde R = La, Nd, Pr), sendo que, para a amostra de La foi estudada a série LaMn2(Si(1-x)Gex)2 (onde x = 0, 0.2, 0.4, 0.6, 0.8, 1), por meio da técnica de correlação angular gama-gama perturbada (CAP), utilizando os núcleos de prova 111In(111Cd) e 140La(140Ce). Além disso, foram realizadas medições para a caracterização estrutural utilizando a técnica de difração de raios X, para a caracterização magnética, a nível macroscópico, através de medições de magnetização, caracterização nuclear utilizando a técnica de espectrometria da radiação gama e um estudo complementar por cálculos de primeiros princípios somente para as amostras de LaMn2Ge2 e LaMn2Si2. A maior contribuição para o comportamento magnético nesses compostos tem origem no ordenamento magnético da sub rede do Mn, que aparece em temperaturas relativamente altas, da ordem de 480 K, sendo que, esses compostos possuem um ordenamento antiferromagnético e próximo a temperatura ambiente passam a possuir um ordenamento ferromagnético. Além disso, para o caso especifico dos compostos PrMn2Ge2 e NdMn2Ge2, abaixo de 40K é observado, também, o ordenamento ferromagnético dos spins da sub rede da terra rara. O núcleo de prova 111In(111Cd) foi utilizado para medir, tanto as interações de quadrupolo elétrico, como de dipolo magnético na rede do Mn (interação matrizmatriz). Esses resultados evidenciaram a transição de fase magnética (antiferromagnética para ferromagnética), sendo que, o campo hiperfino magnético, para as duas fases magnéticas, segue o comportamento da função de Brillouin. Já o núcleo de prova 140La(140Ce) permitiu medir a interação de dipolo magnético, tanto originada pela rede do Mn, como pela rede da terra rara. Mas, nesse caso, pode ser verificado somente o campo hiperfino magnético da fase ferromagnética da rede do Mn. Para a análise dos resultados PAC levou-se em conta uma interação matriz-impureza, pois o íon Ce3+ possui um elétron 4f desemparelhado, que pode contribuir para o campo hiperfino total. O composto LaMn2Si2 apresentou o comportamento do campo hiperfino magnético com a temperatura que segue o comportamento da função de Brillouin. Já os compostos LaMn2Ge2, PrMn2Ge2 e NdMn2Ge2 apresentaram um comportamento anômalo ao da função de Brillouin. Esse comportamento pode ser associado a uma forte hibridização da banda 4f do Ce com a banda 3d do Mn, tal conclusão foi reforçada pelos resultados do DOS (densidade de estados), resultados dos cálculos de primeiros princípios, dos compostos LaMn2Si2 e LaMn2Ge2. / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

In this thesis we investigated two major issues in astrophysical flows: the transport of magnetic fields in highly conducting fluids in the presence of turbulence, and the turbulence evolution and turbulent dynamo amplification of magnetic fields in collisionless plasmas. The first topic was explored in the context of star-formation, where two intriguing problems are highly debated: the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (the so called \"magnetic flux problem\") and the formation of rotationally sustained protostellar discs in the presence of the magnetic fields which tend to remove all the angular momentum (the so called \"magnetic braking catastrophe\"). Both problems challenge the ideal MHD description, usually expected to be a good approximation in these environments. The ambipolar diffusion, which is the mechanism commonly invoked to solve these problems, has been lately questioned both by observations and numerical simulation results. We have here investigated a new paradigm, an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD). We tested the TRD through fully 3D MHD numerical simulations, injecting turbulence into molecular clouds with initial cylindrical geometry, uniform longitudinal magnetic field and periodic boundary conditions. We have demonstrated the efficiency of the TRD in decorrelating the magnetic flux from the gas, allowing the infall of gas into the gravitational well while the field lines migrate to the outer regions of the cloud. This mechanism works for clouds starting either in magnetohydrostatic equilibrium or initially out-of-equilibrium in free-fall. We estimated the rates at which the TRD operate and found that they are faster when the central gravitational potential is higher. Also we found that the larger the initial value of the thermal to magnetic pressure ratio (beta) the larger the diffusion process. Besides, we have found that these rates are consistent with the predictions of the theory, particularly when turbulence is trans- or super-Alfvénic. We have also explored by means of 3D MHD simulations the role of the TRD in protostellar disks formation. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low mass stars. Previous studies showed that an enhanced microscopic diffusivity of about three orders of magnitude larger than the Ohmic diffusivity would be necessary to enable the formation of a rotationally supported disk. However, the nature of this enhanced diffusivity was not explained. Our numerical simulations of disk formation in the presence of turbulence demonstrated the efficiency of the TRD in providing the diffusion of the magnetic flux to the envelope of the protostar during the gravitational collapse, thus enabling the formation of rotationally supported disks of radius ~ 100 AU, in agreement with the observations. The second topic of this thesis has been investigated in the framework of the plasmas of the intracluster medium (ICM). The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action which is known to amplify the magnetic energy until close equipartition with the kinetic energy. This is generally derived employing a collisional MHD model. However, this is poorly justified a priori since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless-MHD description. We have studied here the turbulence statistics and the turbulent dynamo amplification of seed magnetic fields in the ICM using a single-fluid collisionless-MHD model. This introduces an anisotropic thermal pressure with respect to the direction of the local magnetic field and this anisotropy modifies the MHD linear waves and creates kinetic instabilities. Our collisionless-MHD model includes a relaxation term of the pressure anisotropy due to the feedback of the mirror and firehose instabilities. We performed 3D numerical simulations of forced transonic turbulence in a periodic box mimicking the turbulent ICM, assuming different initial values of the magnetic field intensity and different relaxation rates of the pressure anisotropy. We showed that in the high beta plasma regime of the ICM where these kinetic instabilities are stronger, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model in the description of the statistical properties of the turbulence. Also, the amplification of the magnetic energy due to the turbulent dynamo action when considering an initial seed magnetic field is similar to the collisional-MHD model, particularly when considering an instantaneous anisotropy relaxation. The models without any pressure anisotropy relaxation deviate significantly from the collisional-MHD results, showing more power in small-scale fluctuations of the density and velocity field, in agreement with a significant presence of the kinetic instabilities; however, the fluctuations in the magnetic field are mostly suppressed. In this case, the turbulent dynamo fails in amplifying seed magnetic fields and the magnetic energy saturates at values several orders of magnitude below the kinetic energy. It was suggested by previous studies of the collisionless plasma of the solar wind that the pressure anisotropy relaxation rate is of the order of a few percent of the ion gyrofrequency. The present study has shown that if this is also the case for the ICM, then the models which best represent the ICM are those with instantaneous anisotropy relaxation rate, i.e., the models which revealed a behavior very similar to the collisional-MHD description. / Nesta tese, investigamos dois problemas chave relacionados a fluidos astrofísicos: o transporte de campos magnéticos em plasmas altamente condutores na presença de turbulência, e a evolução da turbulência e amplificação de campos magnéticos pelo dínamo turbulento em plasmas não-colisionais. O primeiro tópico foi explorado no contexto de formação estelar, onde duas questões intrigantes são intensamente debatidas na literatura: a necessidade da difusão de fluxo magnético durante o colapso gravitacional de nuvens moleculares, a fim de explicar as intensidades dos campos magnéticos observadas em proto-estrelas (o denominado \"problema do fluxo magnético\"), e a formação de discos proto-estelares sustentados pela rotação em presença de campos magnéticos, os quais tendem a remover o seu momento angular (a chamada \"catástrofe do freamento magnético\"). Estes dois problemas desafiam a descrição MHD ideal, normalmente empregada para descrever esses sistemas. A difusão ambipolar, o mecanismo normalmente invocado para resolver estes problemas, vem sendo questionada ultimamente tanto por observações quanto por resultados de simulações numéricas. Investigamos aqui um novo paradigma, um mecanismo de difusão alternativo baseado em reconexão magnética rápida induzida pela turbulência, que denominamos reconexão turbulenta (TRD, do inglês turbulent reconnection diffusion). Nós testamos a TRD através de simulações numéricas tridimensionais MHD, injetando turbulência em nuvens moleculares com geometria inicialmente cilíndrica, permeadas por um campo magnético longitudinal e fronteiras periódicas. Demonstramos a eficiência da TRD em desacoplar o fluxo magnético do gás, permitindo a queda do gás no poço de potencial gravitacional, enquanto as linhas de campo migram para as regiões externas da nuvem. Este mecanismo funciona tanto para nuvens inicialmente em equilíbrio magneto-hidrostático, quanto para aquelas inicialmente fora de equilíbrio, em queda livre. Nós estimamos as taxas em que a TRD opera e descobrimos que são mais rápidas quando o potencial gravitacional é maior. Também verificamos que quanto maior o valor inicial da razão entre a pressão térmica e magnética (beta), mais eficiente é o processo de difusão. Além disto, também verificamos que estas taxas são consistentes com as previsões da teoria, particularmente quando a turbulência é trans- ou super-Alfvénica. Também exploramos por meio de simulações MHD 3D a influência da TRD na formação de discos proto-estelares. Sob condições MHD ideais, a remoção do momento angular do disco progenitor pelo campo magnético da nuvem pode evitar a formação de discos sustentados por rotação durante a fase principal de acreção proto-estelar de estrelas de baixa massa. Estudos anteriores mostraram que uma super difusividade microscópica aproximadamente três ordens de magnitude maior do que a difusividade ôhmica seria necessária para levar à formação de um disco sustentado pela rotação. No entanto, a natureza desta super difusividade não foi explicada. Nossas simulações numéricas da formação do disco em presença de turbulência demonstraram a eficiência da TRD em prover a diffusão do fluxo magnético para o envelope da proto-estrela durante o colapso gravitacional, permitindo assim a formação de discos sutentados pela rotação com raios ~ 100 UA, em concordância com as observações. O segundo tópico desta tese foi abordado no contexto dos plasmas do meio intra-aglomerado de galáxias (MIA). A amplificação e manutenção dos campos magnéticos observados no MIA são normalmente atribuidas à ação do dínamo turbulento, que é conhecidamente capaz de amplificar a energia magnética até valores próximos da equipartição com a energia cinética. Este resultado é geralmente derivado empregando-se um modelo MHD colisional. No entanto, isto é pobremente justificado a priori, pois no MIA o caminho livre médio de colisões íon-íon é da ordem das escalas dinâmicas, requerendo então uma descrição MHD não-colisional. Estudamos aqui a estatística da turbulência e a amplificação por dínamo turbulento de campos magnéticos sementes no MIA, usando um modelo MHD não-colisional de um único fluido. Isto indroduz uma pressão térmica anisotrópica com respeito à direção do campo magnético local. Esta anisotropia modifica as ondas MHD lineares e cria instabilidades cinéticas. Nosso modelo MHD não-colisional inclui um termo de relaxação da anisotropia devido aos efeitos das instabilidades mirror e firehose. Realizamos simulações numéricas 3D de turbulência trans-sônica forçada em um domínio periódico, mimetizando o MIA turbulento e considerando diferentes valores iniciais para a intensidade do campo magnético, bem como diferentes taxas de relaxação da anisotropia na pressão. Mostramos que no regime de plasma com altos valores de beta no MIA, onde estas instabilidades cinéticas são mais fortes, uma rápida taxa de relaxação da anisotropia produz resultados similares ao modelo MHD colisional na descrição das propriedades estatísticas da turbulência. Além disso, a amplificação da energia mangética pela ação do dínamo turbulento quando consideramos um campo magnético semente, é similar ao modelo MHD colisional, particularmente quando consideramos uma relaxação instantânea da anisotropia. Os modelos sem qualquer relaxação da anisotropia de pressão mostraram resultados que se desviam significativamente daqueles do MHD colisional, mostrando mais potências nas flutuações de pequena escala da densidade e velocidade, em concordância com a presença significativa das instabilidades cinéticas nessas escalas; no entanto, as flutuações do campo magnético são, em geral, suprimidas. Neste caso, o dínamo turbulento também falha em amplificar campos magnéticos sementes e a energia magnética satura em valores bem abaixo da energia cinética. Estudos anteriores do plasma não-colisional do vento solar sugeriram que a taxa de relaxação da anisotropia na pressão é da ordem de uma pequena porcentagem da giro-frequência dos íons. O presente estudo mostrou que, se este também é o caso para o MIA, então os modelos que melhor representam o MIA são aqueles com taxas de relaxação instantâneas, ou seja, os modelos que revelaram um comportamento muito similar à descrição MHD colisional.

Les méthodes d'induction électromagnétique permettent de caractériser la conductivité électrique des matériaux, dont les corps planétaires telluriques, depuis les couches superficielles de la croûte jusqu'aux zones les plus internes, dans le manteau inférieur. Pour une source de champ électromagnétique donnée, des courants sont induits dans les matériaux qui y sont soumis. Avec l'essor des données magnétiques satellitaires, de nouvelles méthodes d'analyse des données magnétiques permettent d'obtenir des images unidimensionnelles de la structure électrique de ces corps car la structure spatio-temporelle de la source électromagnétique en est bien connue. Les travaux de mon doctorat ont eu pour but de mettre en place une nouvelle méthode d'analyse permettant de déterminer des modèles de structure interne globaux pour n'importe quel corps du système solaire pour lequel on dispose de longues séries temporelles magnétiques satellitaires. Après avoir testé cette méthode sur des modèles synthétiques et l'avoir appliqué au cas de données réelles terrestre pour lesquelles des études d'induction électromagnétiques antérieurs permettent d'avoir un a priori sur le modèle de conductivité électrique attendu, nous avons obtenu les premiers modèles de conductivité électrique martien en utilisant les données magnétiques du satellite Mars Global Surveyor. Ces résultats nous ont permis de valider des modèles de structure interne antérieurs établis à partir d'analyses géochimiques et minéralogiques des météorites martiennes. Cette méthode innovante est aujourd'hui la seule capable d'obtenir une image électrique des manteaux telluriques à partir de données magnétiques satellitaires pour des corps autres que la Terre ou la Lune et pour lesquels aucun a priori sur la structure spatio-temporelle du champ électromagnétique inducteur externe n'est nécessaire. / My Ph.D. work consists in the investigation of satellite magnetic data to infer the deep internal conductivity distribution. I developed a new global electromagnetic induction method applied to planetary magnetic datasets without strong a priori hypothesis on the external inducing source field. My method is based on a spectral correction of gapped data magnetic time series to restore the time spectral content of the source field. This external source depends on the planetary environment and is therefore different for each planetary bodies. The method aims at recovering with a maximum accuracy internal and external spherical harmonic coefficients of transients fields, whose ratio is used as a transfer function to retrieve the internal distribution of electrical conductivity. While for the Earth, a good proxy of the source field activity is the Dst index, no such proxy exists for other planets. Hence, for our study of Mars transient magnetic field from MGS, one of the major part of my work is the determination of an appropriate continuous proxy for the external variability. On Earth the external electromagnetic source is well known, and may be described by a spherical harmonic geometry dominated by the dipole term. This source field may be characterized using a magnetic activity index named the Dst index. The method has been tested on synthetic data generated within the framework of SWARM mission. This mission consists of a 3 satellites constellation. One of the main objectives is to infer the 3D electrical distribution in the deep Earth. SWARM synthetic data consist in a time series of spherical harmonic (SH) coefficients, external and internal, generated from a simple non-realistic 3D model. In this model, several regional and local conductors, in a radially symmetric 3 layers model have been embedded. Using this dataset, our method give satisfactory results. We have been able to obtain the external and internal SH coefficients - for the first SH degree, which is known to be the most energetic degree of the external source - using only one of the 3 synthetic time series. Then, the method has been used on real data from Ørsted. In this case, we had to pre-process the data to correct from ionospheric and aligned currents contributions. We developed a statistical analysis to remove the ionospheric field using 2 geomagnetic indices : AL and Kp. Hence, we have enlarged data toward higher and lower latitudinal zones than what has been done in previous works. Finally, we have been able to obtain 1D conductivity models, which fits reasonably with existing conductivity data in the deep Earth. Finally, we worked on Mars Global Surveyor (MGS) data. One of the most time consuming parts of this work was the determination of an appropriate continuous proxy for the external variability in the vicinity of Mars. Without any measurements of the IMF (Interplanetary Magnetic Field) during MGS sciences acquisition, we have used ACE (Advanced Composition Explorer) data. This satellite orbits around the L1 point of the Sun-Earth system, measuring solar wind magnetic characteristics. We have time-shifted ACE data to Mars position for 4 temporal windows where Mars and Earth were closed to the same Parker's spiral's arm, and finally determined a proxy explaining the major part of the variability observed in Mars data. Despite numerous gaps in MGS data, we have been able to establish the 1D conductivity distribution, fitting reasonably existing geochemical models. Although the method may be unstable for some cases, we obtained satisfactory results for in depth conductivity of the planet.

L’IRM quantitative recouvre l’ensemble des méthodes permettant de mesurer des paramètres physiques accessibles en Résonance Magnétique Nucléaire. Elle offre un bénéfice par rapport à l’imagerie en pondération classiquement utilisée, notamment pour la détection, la caractérisation physiopathologique mais aussi pour le suivi thérapeutique des pathologies. Malgré ce potentiel avéré connu de longue date, ces méthodes restent peu utilisées dans la routine clinique. La raison principale est la longueur des acquisitions par rapport à l’approche classique. Les paramètres physiques que nous souhaitons étudier plus particulièrement sont le temps de relaxation longitudinal (T₁), transversal (T₂), le coefficient de diffusion apparent (ADC), et la densité de protons (DP). Malgré la possibilité d’atteindre une meilleure qualité d’images, ces cartographies in vivo sont quasiment inexistantes dans la littérature au-delà de 3T car leur implémentation nécessite de surmonter un certain nombre de limites spécifiques aux IRM ultra-haut champs (UHF). Au travers de ce projet de thèse, une méthode d’imagerie quantitative basée sur les états de configurations (QuICS) a été implémentée, pour déterminer ces paramètres quantitatifs de façon simultanée sous fortes contraintes propres aux UHF. L’approche a été optimisée dans le but d’obtenir des cartographies fiables et rapides. Le potentiel de la méthode a été démontré dans un premier temps in vitro sur un noyau tel que le sodium démontrant des propriétés complexes à cartographier. Puis dans un second temps, des acquisitions ont été réalisées sur proton, in vivo, en un temps d’acquisition compatible avec une utilisation en routine clinique à 7T. L’application d’une telle méthode d’IRM quantitative à UHF sur des populations permettra d’ouvrir de nouvelles voies d’études pour le futur. / Quantitative MRI refers to methods able to measure different physical parameters accessible in Nuclear Magnetic Resonance. It offers benefits compared to weighting imaging commonly used, for the detection, the pathophysiological characterization but also for the therapeutic follow-up of pathologies for example. Despite this long-established potential, these methods remain little used in clinical routine. The main reason is the long acquisition time compared to the classical approach. The physical parameters that we will study more particularly are the longitudinal (T₁), transverse (T₂) relaxation time, the apparent diffusion coefficient (ADC), and the proton density (DP). Despite the possibility to achieve a better image quality, these in vivo mappings are virtually non-existent in the literature beyond 3T because their implementation requires overcom-ing a number of specific ultra-high-field (UHF) MRI limits. Through this thesis project, a Quantitative Imaging method using Configuration States (QuICS) was implemented under strong UHF constraints, to determine these parameters simultaneously. The technique has been optimized to obtain fast and reliable maps. The potential of the method was first demon-strated in vitro on a nucleus such as sodium, exhibiting complex properties. As a second step, acquisitions were performed in proton, in vivo, in an clinically-relevant acquisition time, compatible with a routine use at 7T for population imaging. The application of such a method of quantitative MRI to UHF will open new research possibilities for the future.

The present thesis is devoted to the investigation of the magnetocaloric effect (MCE) by direct measurements in pulsed and quasi-static magnetic fields as well as by analyzing specific-heat data taken in static magnetic fields. The emphasis is on the direct measurement of the adiabatic temperature change Tad in pulsed magnetic fields, because the pulsed-field data allow for an analysis of the sample-temperature response to the magnetic field on a time scale of 10 to 100 ms, which is on the order of typical operation frequencies (10 - 100 Hz) of magnetocaloric cooling devices. Besides extending the accessible magneticfield range to beyond 70 T, the short pulse duration provides nearly adiabatic conditions during the measurement. In this work, the magnetocaloric properties of various types of solids are investigated: Gadolinium (Gd) with a second-order transition, Ni50Mn35In15 with multiple magnetic transitions, and La(Fe,Si,Co)13 compounds with first and second-order transitions, depending on the Co concentration. The adiabatic temperature change of a polycrystalline Gd sample has been measured in pulsed magnetic fields up to 70 T and also in quasi-static fields up to 2 T. A very large adiabatic temperature change of Tad 60 K is observed near the Curie temperature (TC = 294 K) for a field change of 70 T. In addition, we find that this maximum temperature change grows with H2=3. We have studied the MCE in the shape-memory Heusler alloy Ni50Mn35In15 by direct measurements in pulsed magnetic fields up to 6 and 20 T. The results obtained for 6 T pulses are compared with data extracted from specific-heat experiments. We find a saturation of the inverse MCE, related to the firstorder martensitic transition, with a maximum adiabatic temperature change of Tad = 7 K at 250 K and a conventional field-dependent MCE near the second-order ferromagnetic transition in the austenitic phase. Our results disclose that in shape-memory alloys the different contributions to the MCE and hysteresis effects around the martensitic transition have to be carefully considered for future cooling applications. Finally, a comparative study of the magnetic and magnetocaloric properties of La(Fe,Si,Co)13 alloys is presented by discussing magnetization, Tad, specificheat, and magnetostriction measurements. The nature of the transition can be changed from first to second order as well as the temperature of the transition can be tuned by varying the Co concentration. The MCE of two samples with nominal compositions of LaFe11:74Co0:13Si1:13 and LaFe11:21Co0:65Si1:11 have been measured in pulsed magnetic fields up to 50 T. We find that LaFe11:74Co0:13Si1:13 with a first-order transition (TC = 198 K) shows half of the net MCE already at low fields (2-10 T). Whereas the MCE of LaFe11:21Co0:65Si1:11 with secondorder transition (TC = 257 K) grows gradually. The MCE in both compounds reaches almost similar values at a field of 50 T. The MCE results obtained in pulsed magnetic fields of 2 T are in good agreement with data from quasistatic field measurements. The pulsed-field magnetization of both compounds has been measured in fields up to 60 T under nearly adiabatic conditions and compared to steady-field isothermal measurements. The differences between the magnetization curves obtained under isothermal and adiabatic conditions give the MCE via the crossing points of the adiabatic curve with the set of isothermal curves. For LaFe11:74Co0:13Si1:13, a S - T diagram has been constructed from specific-heat measurements in static fields, which is used to extract the MCE indirectly. Magnetostriction measurements are carried out for two compounds in both static and pulsed magnetic fields. For LaFe11:74Co0:13Si1:13, the strain shows a sharp increase. However, due to cracks appearing in the sample an irreversible magneto-volume effect of about 1% is observed in pulsed magnetic fields. Whereas for LaFe11:21Co0:65Si1:11 the data show a smooth increase of the sample length up to 60 T, and a 1.3% volume increase is obtained. We also find that magnetizing the latter sample in the paramagnetic state is tightly bound to the volume increase and this, likewise for the former sample, gives the main contribution to the entropy change.

Les Jupiters chaudes sont des exoplanètes possédant des caractéristiques uniques. En raison de leur proximité avec leur étoile hôte elles présentent une non-symétrie remarquable. Cette proximité provoquant la rotation synchrone force un côté de la planète à toujours faire face à l'étoile et l'autre à être plongé dans une nuit perpétuelle. Cette géométrie donne lieu à une différence d'allant de 200 K jusqu'à 2000 K entre les deux côtés de la planète, engendrant des écoulements zonaux pouvant atteindre des vitesses de l'ordre du km/s afin de redistribuer la chaleur. Le point chaud, le point le plus chaud de la planète, est un témoin de ces vents intenses. Les observations et les simulations hydrodynamiques montrent que les écoulements zonaux se font d'ouest en est. Cependant, les observations de deux planètes ne se conforment pas aux prédictions. En effet, CoRoT-2 b et HAT-P-7 b montrent des points chauds à l'ouest. L'explication la plus répandue est que le champ magnétique de ces planètes, en interaction avec leur atmosphère partiellement ionisée, peut renverser la direction des écoulements zonaux, si ce champ est assez puissant. Une diffusivité magnétique variable dans l'espace peut générer localement des champs magnétiques lorsque son gradient s'aligne correctement avec le courant électrique. Nous présentons ici un modèle magnétohydrodynamique en une dimension possédant une diffusivité magnétique dépendante de la température dans le plan équatorial dans le contexte de Jupiters chaudes. Les résultats des simulations présentent des oscillations torsionnelles de type alfvéniques reflétant les effets non linéaires dus au couplage des équations aux dérivées partielles de la magnétohydrodynamique et de la température avec la diffusivité magnétique dépendante de la température. Nous explorons un espace des paramètres afin d'établir l'influence de ceux-ci sur les oscillations. Nous avons aussi développé un modèle local afin de dériver des équations analytiques nous permettant de mieux comprendre les résultats observés en plus de comparer les résultats du modèle en une dimension avec ceux du modèle local. Nous finissons par établir que les oscillations générées par notre modèle en une dimension possèdent des périodes équivalentes allant de 225 à 473 jours et des déplacements longitudinaux équivalant à quelques degrés jusqu'à environ 40° pour une planète de la taille de Jupiter. Ces intervalles de périodes et de déplacements sont encourageants, puisque cela signifie que les oscillations pourraient être observées. / Hot Jupiters are exoplanets with unique features. Due to their proximity to their host stars, they show remarkable non-symmetry. This proximity with the star causes tidal locking, meaning one side of the planet is always exposed to intense radiation from its host and the other side is immersed in a perpetual night. This geometry means there is a difference of temperature ranging from 200 K up to 2000 K between the day and night side. This gradient in temperature induces zonal winds that can reach the order of 1 km/s to redistribute heat to the night side. The hot spot is the hottest spot of the planet and is a telltale of these strong winds. Observations and hydrodynamic numerical simulations show that zonal winds on these planets go eastward. However, two recent observations are showing westward winds. These planets are CoRoT-2 b and HAT-P-7 b. The most common explanation to this contradiction is that the magnetic field, which is interacting with the partially ionized atmosphere, can reverse these winds. It was previously shown that a magnetic diffusivity varying in space can locally generate magnetic fields when its gradient aligns correctly with the electric current density. We present here a one-dimensional magnetohydrodynamic model with a temperature-dependent magnetic diffusivity in the equatorial plane in the context of hot Jupiters. The simulations develop growing torsional alfvénic oscillations due to the non-linear coupling of the magnetohydrodynamics and the temperature partial differential equations and the temperature-dependent magnetic diffusivity. We explore the parameter space and study their influence on the oscillations. We have also developed a local model in order to derive analytical equations characterizing these waves and compare its results with the results of the one-dimensional model. We end by calculating the corresponding periods and longitudinal displacement of the one-dimension model oscillations for a Jupiter-sized planet. The periods correspond to an interval from 225 to 473 days and the displacements range from a few degrees up to 40°. This means that the oscillations could be observed with a few orbits.

Fundamental magnetic properties have been studied in RFexAl12-x (R - heavy rare-earth element) compounds with the tetragonal crystal structure of the ThMn12 type. In the homogeneity range 4 ≤ x ≤ 6, exchange interactions and magnetocrystalline anisotropy of the Fe sublattice are significantly modified as found in LuFexAl12-x with non-magnetic Lu. With increasing Fe content, antiferromagnetic exchange interactions weaken and ferromagnetic strengthen. The change of the type of magnetic ordering occurs in the vicinity of x = 5. The Fe sublattice displays an easy-plane magnetic anisotropy that weakens with growing Fe concentration. RFe5Al7 compounds with magnetic R are highly anisotropic ferrimagnets with Curie temperatures from 193 to 262 K. Spontaneous and field-induced magnetic phase transitions of anisotropic and exchange nature have been found in RFe5Al7 by measurements of magnetization and sound propagation. The transitions provide quantitative information on the R-Fe inter-sublattice exchange. A correlation between the R element and the fundamental magnetic properties has been found for RFe5Al7. The Fe magnetic moment is near 8 µB per formula unit. The compounds with R - Tb, Dy, Ho and Er have a compensation of the rare- earth and iron sublattice magnetizations. The compensation point and Curie...

We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.

Dans le cadre de la fusion par confinement inertiel, une capsule contenant le combustible de deutérium-tritium est implosée soit par irradiation laser (attaque directe, interaction laser – cible de numéro atomique faible), soit par un rayonnement de corps noir émis par une cavité convertissant le rayonnement laser (attaque indirecte, interaction laser – cible de numéro atomique élevé).Dans les deux cas, une modélisation correcte du transport électronique est cruciale pour avoir des simulations hydro-radiatives prédictives. Cependant, il a été montré très tôt que les hypothèses d'un mécanisme de transport linéaire ne sont pas applicables dans le cadre de l'irradiation d'une cible solide par un laser de puissance (I~10^14 W/cm²). Cela est dû d'une part à des gradients de température très importants (effets cinétiques dits « non-locaux ») ainsi qu'à la présence d'un champ magnétique auto-généré par effet thermo-électrique. Enfin, le flux de chaleur et le champ magnétique sont fortement couplés au travers de deux mécanismes : le transport du champ magnétique par le flux de chaleur (effet Nernst) et la rotation et inhibition du flux de chaleur par la magnétisation du plasma (effet Righi-Leduc).Dans le présent manuscrit, nous commencerons par exposer les différents modèles de transport électronique, et en particulier le modèle non-local avec champ magnétique, implémenté dans le code hydro-radiatif FCI2. Par la suite, nous chercherons à valider ce modèle par des comparaisons avec un code cinétique, puis avec une expérience lors de laquelle le champ magnétique a été mesuré par radiographie proton. Cela fait, nous utiliserons le code FCI2 pour expliquer la source et le transport du champ, ainsi que son effet sur l'interaction.Enfin, nous étudierons la reconnexion du champ magnétique, lors de l'irradiation d'une cible par deux faisceaux lasers. / In the framework of the inertial confinement fusion, a pellet filled with the deuterium-tritium fuel is imploded, either through laser irradiation (direct drive, laser – low atomic number target interaction) or by the black body radiation from a cavity converting the laser radiation (indirect drive, laser – high atomic number target interaction).In both cases, a correct modeling of the electron transport is of first importance in order to have predictive hydro-radiative simulations. Nonetheless, it has been shown early on that the hypothesis of the linear transport are not valid in the framework of a solid target irradiated by a high power laser (I~1014 W/cm²). This is due in part to very steep temperature gradients (kinetic effects, so-called « non-local ») and because of a magnetic field self-generated through the thermo-electric effect. Finally, the heat flux and the magnetic field are strongly coupled through two mecanisms: the advection of the field with the heat flux (Nernst effect) and the rotation and inhibition of the heat flux by the plasma's magnetization (Righi-Leduc effect).In this manuscript, we will first present the various electron transport models, particularly the non-local with magnetic field model included in the hydro-radiative code FCI2. Following, in order to validate this model, we will compare it first against a kinetic code, and then with an experiment during which the magnetic field has been probed through proton radiography. Once the model validated, we will use FCI2 simulations to explain the source and transport of the field, as well as its effect on the interaction.Finally, the reconnection of the magnetic field, during the irradiation of a solid target by two laser beams, will be studied.

The origin of Ultra-High-Energy Cosmic Rays (UHECRs) is still an open question in Astrophysics. The scope of this thesis is the study of the termination shocks in the jets of radiogalaxies 3C 105, 3C 227, and 3C 445 as cosmic rays accelerators. We assume the diffusive shock acceleration (DSA) and that the maximum energy is determined by the escape of cosmic rays downstream of the shock due to the magnetization. We find maximum achievable energy of non-thermal electrons and protons, which appeared to be only~TeV. Therefore the reverse shocks of AGNs' jets are not the sources of UHECRs. We also implement the model, where the scattering centers for the DSA emerge from the non- resonant hybrid instabilities in plasma. We take into account the results from the numerical simulations and observations that indicate the amplification of the magnetic field by orders of magnitude, and we obtain results that qualitatively fit the simulation outcome.