Modelo não linear de dínamo solar unidimensional

• Ano de defesa: 2020
• Autor(a) principal: Lucas Mourão Vieira Montese
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; Brasil; ICX - DEPARTAMENTO DE FÍSICA; Programa de Pós-Graduação em Física; UFMG
• Palavras-chave em Português: Dínamo de campo médio; Sol; Manchas solares; Atividade magnética; Helicidade magnética; Campo magnético; Astrofísica
• Link de acesso: http://hdl.handle.net/1843/35529

In this work we study the generation of the large scale magnetic fields observed in cosmic objects using analytical and numerical turbulent mean-field dynamo models. We use a simplified, one-dimensional formulation, with fixed values for differential rotation, ∇Ω, the turbulent α effect and the turbulent magnetic diffusivity ηt, the so-called dynamo coefficients. Dynamo models α2, αΩ and α2Ω describing the evolution of the toroidal and poloidal components of the magnetic field are solved. It is expected that for the case of the Sun, for example, these equations have supercritical solutions, causing exponentially growing magnetic fields. Therefore, we study two different mechanisms of saturation which quench the contribution of the α effect, one static (algebraic) and the other dynamic. While the first formalism is based on heuristic considerations, the second carries the concept of the conservation of magnetic helicity in ideal MHD. The two formulations depend inversely on the magnetic Reynolds number, leading to catastrophic quenching of the dynamo, since in these objects, Rm >> 1. This quenching is alleviated when a magnetic helicity flux is included in the dynamic equation of α. The simulations describe the behavior of the magnetic field by varying the dynamo coefficients as well as the coefficient that controls the magnetic quenching flux, κα. For the α2 model, we find that the average amplitude of the field decreases according to the scaling relation, Bmedia ∝ R−0.5m, which agrees with previous results in literature. For the models αΩ and α2Ω, the decay is proportional to R−0.8m, possibly due to the low dimensionality of the model. In all three cases, the inclusion of a diffusive flux of the magnetic helicity allows the dynamo to sustain considerable values of the field, even for Rm = 109. The magnetic cycle period as a function of the dynamo number, for the models αΩ and α2Ω behaves as, T ∼ D0.5. As a consequence of magnetic helicity diffusion, a second periodicity is observed which encompasses the main cycle. The α2 models do not exhibit any periodicity. For dynamo numbers >> 1, as expected in astrophysical objects, models αΩ and α2Ω have divergent results regarding the cycle period. Applying our results to values of ∇Ω, α and ηt compatible with the solar interior, our simulations are able to reproduce the period of 22 years, as well as the cycles of 2 and 80 years, depending on the region where they are analyzed.