This thesis considers the properties of MHS equilibria formed through non-resistive MHD relaxation of analytical non-potential magnetic field models, which contain two null points connected by a generic separator. Four types of analytical magnetic fields are formulated, with different forms of current. The magnetic field model which has a uniform current directed along the separator, is used through the rest of this thesis to form MHS equilibria and to study reconnection. This magnetic field, which is not force-free, embedded in a high-beta plasma, relaxes non-resistively using a 3D MHD code. The relaxation causes the field about the separator to collapse leading to a twisted current layer forming along the separator. The MHS equilibrium current layer slowly becomes stronger, longer, wider and thinner with time. Its properties, and the properties of the plasma, are found to depend on the initial parameters of the magnetic field, which control the geometry of the magnetic configuration. Such a MHS equilibria is used in a high plasma-beta reconnection experiment. An anomalous resistivity ensures that only the central strong current in the separator current layer is dissipated. The reconnection occurs in two phases characterised by fast and slow reconnection, respectively. Waves, launched from the diffusion site, communicate the loss of force balance at the current layer and set up flows in the system. The energy transport in this system is dominated by Ohmic dissipation. Several methods are presented which allow a low plasma-beta value to be approached in the single-separator model. One method is chosen and this model is relaxed non-resistively to form a MHS equilibrium. A twisted current layer grows along the separator, containing stronger current than in the high plasma-beta experiments, and has a local enhancement in pressure inside it. The growth rate of this current layer is similar to that found in the high plasma-beta experiments, however, the current layer becomes thinner and narrower over time.

The dynamics of the solar corona are dominated by the magnetic field which creates its structure. The magnetic field in most of the corona is ‘frozen’ to the plasma very effectively. The exception is in small localised regions of intense current concentrations where the magnetic field can slip through the plasma and a restructuring of the magnetic field can occur. This process is known as magnetic reconnection and is believed to be responsible for a wide variety of phenomena in the corona, from the rapid energy release of solar flares to the heating of the high-temperature corona. The coronal field itself is three-dimensional (3D), but much of our understanding of reconnection has been developed through two-dimensional (2D) models. This thesis describes several models for fully 3D reconnection, with both kinematic and fully dynamic models presented. The reconnective behaviour is shown to be fundamentally different in many respects from the 2D case. In addition a numerical experiment is described which examines the reconnection process in coronal magnetic flux tubes whose photospheric footpoints are spun, one type of motion observed to occur on the Sun. The large-scale coronal field itself is thought to be generated by a magnetohydrodynamic dynamo operating in the solar interior. Although the dynamo effect itself is not usually associated with reconnection, since the essential element of the problem is to account for the presence of large-scale fields, reconnection is essential for the restructuring of the amplified small-scale flux. Here we examine some simple models of the solar-dynamo process, taking advantage of their simplicity to make a full exploration of their behaviour in a variety of parameter regimes. A wide variety of dynamic behaviour is found in each of the models, including aperiodic modulation of cyclic solutions and intermittency that strongly resembles the historic record of solar magnetic activity.

No description available.

Significant X-ray variability and flaring has been observed from Sgr A* but is poorly understood from a theoretical standpoint. We perform general relativistic magnetohydrodynamic simulations that take into account a population of non-thermal electrons with energy distributions and injection rates that are motivated by PIC simulations of magnetic reconnection. We explore the effects of including these non-thermal electrons on the predicted broadband variability of Sgr A* and find that X-ray variability is a generic result of localizing non-thermal electrons to highly magnetized regions, where particles are likely to be accelerated via magnetic reconnection. The proximity of these high-field regions to the event horizon forms a natural connection between IR and X-ray variability and accounts for the rapid timescales associated with the X-ray flares. The qualitative nature of this variability is consistent with observations, producing X-ray flares that are always coincident with IR flares, but not vice versa, i.e., there are a number of IR flares without X-ray counterparts.

Made available in DSpace on 2016-04-25T20:20:01Z (GMT). No. of bitstreams: 1 Abel Menezes Filho.pdf: 3516626 bytes, checksum: 16d38d1a4c30b5388fbfbd87fba6f4c8 (MD5) Previous issue date: 2010-12-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / On the basis of the knowledges are fragmented and placed in watertight compartments, this thesis proposes that art, by its transdisciplinary characteristics, may contribute to bridging, to make reconnections, entanglement. The complex thinking is the inspiration and literature was the method chosen to shows this possible dialogue. The narrative is woven by four characters, two women and two men, who proposes to write a collective book. The characters lives in different cities and began the dialogue by e-mails. The model chosen was the caduceus of Hermes, a bat where two snakes interwine , in seven vertexes. Unfolded in the plane there are three narratives, parallel, autonomous and simultaneously intertwined. The side snakes is the form I found to dialogue with the authors with more fluidity and break with the usual understanding of notes, which support or complement the main text, often by rote. Caduceu s doublé helix has resonances with the genetic code, that from four letters, combined in pairs, in different extents, create all forms of life. Although any of the four characters have their predominant thematic fields, the knowledges exchange became possible and the model suited as well / A partir da compreensão de que os saberes estão fragmentados e dispostos em compartimentos estanques, esta tese propõe que a arte, por suas características transdisciplinares pode contribuir para fazer pontes, religações, entrelaçamentos. O pensamento complexo é a inspiração e a literatura foi o método escolhido para mostrar esse possível diálogo. A narrativa é tecida por quatro personagens, duas mulheres e dois homens, que se propõem a escrever um livro coletivo. Os personagens moram em cidades diferentes e começam o diálogo por e-mails. O modelo escolhido foi o caduceu de Hermes, um bastão em que se entrelaçam duas serpentes, em sete vértices. Desdobradas no plano existem três narrativas, paralelas, autônomas e, simultaneamente entrelaçadas. As serpentes laterais é a maneira que encontrei para dialogar com os autores com mais fluidez e romper com a compreensão habitual de notas, que sustentam ou complementam o texto principal, muitas vezes de forma mecânica. A dupla espiral do caduceu tem ressonâncias com o código genético, que a partir de quatro letras, combinadas ao pares, em extensões diferentes, criam todas as formas de vida. Embora cada um dos quatro personagens tenha seus campos temáticos predominantes, foi possível a troca de saberes e o modelo mostrou-se adequado

Made available in DSpace on 2016-04-26T14:52:54Z (GMT). No. of bitstreams: 1 Abel Menezes Filho.pdf: 3516626 bytes, checksum: 16d38d1a4c30b5388fbfbd87fba6f4c8 (MD5) Previous issue date: 2010-12-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / On the basis of the knowledges are fragmented and placed in watertight compartments, this thesis proposes that art, by its transdisciplinary characteristics, may contribute to bridging, to make reconnections, entanglement. The complex thinking is the inspiration and literature was the method chosen to shows this possible dialogue. The narrative is woven by four characters, two women and two men, who proposes to write a collective book. The characters lives in different cities and began the dialogue by e-mails. The model chosen was the caduceus of Hermes, a bat where two snakes interwine , in seven vertexes. Unfolded in the plane there are three narratives, parallel, autonomous and simultaneously intertwined. The side snakes is the form I found to dialogue with the authors with more fluidity and break with the usual understanding of notes, which support or complement the main text, often by rote. Caduceu s doublé helix has resonances with the genetic code, that from four letters, combined in pairs, in different extents, create all forms of life. Although any of the four characters have their predominant thematic fields, the knowledges exchange became possible and the model suited as well / A partir da compreensão de que os saberes estão fragmentados e dispostos em compartimentos estanques, esta tese propõe que a arte, por suas características transdisciplinares pode contribuir para fazer pontes, religações, entrelaçamentos. O pensamento complexo é a inspiração e a literatura foi o método escolhido para mostrar esse possível diálogo. A narrativa é tecida por quatro personagens, duas mulheres e dois homens, que se propõem a escrever um livro coletivo. Os personagens moram em cidades diferentes e começam o diálogo por e-mails. O modelo escolhido foi o caduceu de Hermes, um bastão em que se entrelaçam duas serpentes, em sete vértices. Desdobradas no plano existem três narrativas, paralelas, autônomas e, simultaneamente entrelaçadas. As serpentes laterais é a maneira que encontrei para dialogar com os autores com mais fluidez e romper com a compreensão habitual de notas, que sustentam ou complementam o texto principal, muitas vezes de forma mecânica. A dupla espiral do caduceu tem ressonâncias com o código genético, que a partir de quatro letras, combinadas ao pares, em extensões diferentes, criam todas as formas de vida. Embora cada um dos quatro personagens tenha seus campos temáticos predominantes, foi possível a troca de saberes e o modelo mostrou-se adequado

Abstract This thesis examines magnetic reconnection in the solar corona. Magnetic reconnection is the only mechanism which allows the magnetic topology of magnetized plasmas to be changed. Many of the dynamic processes in the Sun's atmosphere are believed to be driven by magnetic reconnection and studying the behaviour of such phenomena is a key step to understanding the reconnection mechanism. In Chapters 1 to 3, we discuss the physical and mathematical framework on which current magnetohydrodynamic reconnection models are based. The aim of the thesis is to investigate theoretical models of magnetic reconnection using variety of analytic and numerical techniques within the theoretical frame work of magnetohydrodynamics (MHD). In Chapter 4 we use a line-tied X-point collapse model for compressible plasmas to investigate the role of viscosity on the energy release mechanism. This model also provides the basis for the investigation of Chapter 5 which explores the impact of Hall currents in the transient X-point energy dissipation. Chapter 6 is concerned with how reconnection is modified in the presence of generalized Ohm's law which includes both Hall current and electron inertia contributions. In contrast to the closed X-point collapse geometry adopted for compressible plasmas previously, we find it more convenient to explore this problem using an open incompressible geometry in which plasma is continually entering and exiting the reconnection region. Specially, we find the scaling of the Hall-MHD system size analytically, rather than numerically as in the X-point problem of Chapter 5. Chapter 7 summarizes the results of investigations in Chapters 4, 5 and 6.

This thesis is focused on the role of Alfvén waves in the energy transformation and transport in the magnetosphere. Different aspects of Alfvén wave generation, propagation and dissipation are considered. The study involves analysis of experimental data from the Freja, Polar and Cluster spacecraft, as well as theoretical development. An overview of the linear theory of Alfvén waves is presented, including the effects of fnite parallel electron inertia and fnite ion gyroradius, and nonlinear theory is developed for large amplitude Alfvén solitons and structures. The methodology is presented for experimental identification of dispersive Alfvén waves in a frame moving with respect to the plasma, which facilitates the resolution of the space-time ambiguity in such measurements. Dispersive Alfvén waves are identified on field lines from the topside ionosphere up to the magnetopause and it is suggested they play an important role in magnetospheric physics. One of the processes where Alfvén waves are important is the establishment of the field aligned current system, which transports the energy from the reconnection regions at the magnetopause to the ionosphere, where a part of the energy is dissipated. The main mechanism for the dissipation in the top-side ionosphere is related to wave-particle interactions leading to particle energization/heating. An observed signature of such a process is the presence of parallel energetic electron bursts associated with dispersive Alfvén waves. The accelerated electrons (electron beams) are unstable with respect to the generation of high frequency plasma wave modes. Therefore this thesis also demonstrates an indirect coupling between low frequency Alfvén wave and high frequency oscillations.

In this thesis the effect of the Hall term in the generalised Ohm's law on Alfvén (shear) and fast wave propagation and dissipation in the ion cyclotron frequency range is investigated. The damping of an initially Gaussian field perturbation in a uniform Hall MHD plasma is treated analytically. Subsequently a 2D Lagrangian remap code (Lare2d) is used to study the damping and phase mixing of initially Gaussian field perturbations and a harmonic series of boundary-driven perturbations in a uniform field (in the presence of a transverse equilibrium density gradient). The same code is then used to study a range of initially shear and fast-wave perturbations in the vicinity of a magnetic X-type null point. The magnetic energy associated with an initially Gaussian field perturbation in a uniform resistive plasma is shown to decay algebraically at a rate that is unaffected by the Hall term to leading order in kδ where k is wavenumber and δ is ion skin depth. A similar decay law applies to whistler perturbations in the limit kδ>>>1. We demonstrate that in both geometries considered, the inclusion of the Hall term reduces the effectiveness of phase-mixing in plasma heating. The reduction in the damping rate in the uniform field (non-uniform density) cases, arising from dispersive effects, tends to zero in both the weak and strong phase mixing limits. In the Hall MHD X-point case, minimal reductions are seen for initially shear wave pulses, suggesting that little or no phase-mixing takes place. Nonlinear fast wave pulses which interact with the initial X-point destabilise the local field sufficiently to generate multiple null pairs; subsequent oscillatory current sheet behaviour appears unaffected by earlier differences between the MHD and Hall MHD cases.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19091号 / エネ博第315号 / 新制||エネ||64 / 32042 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 岸本 泰明, 教授 前川 孝, 教授 中村 祐司 / 学位規則第4条第1項該当