The Earth's magnetic field is generated in its fluid outer core through dynamo action. In this process, convection and differential rotation of an electrically conducting fluid maintain the magnetic field against its ohmic decay. Using numerical models, we can investigate planetary dynamo processes and the importance of various core properties on the dynamo. In this thesis, I use numerical dynamo models in Earth-like geometry in order to understand the influence of inner core electrical conductivity and the choice of thermal and velocity boundary conditions on the resulting magnetic field. I demonstrate how an electrically conducting inner core can reduce the frequency of reversals and produce axial-dipolar dominated fields in our models. I also demonstrate that a strong planetary magnetic field intensity does not imply that the dynamo operates in the strong field regime as is usually presumed. Through a scaling law analysis, I find that irrespective of the choice of thermal or velocity boundary conditions, the available power determines the magnetic and velocity field characteristics like the field strength, polarity and morphology. Also, whether a dynamo model is in a dipolar, transitional or multipolar regime is dependent on the force balance in the model. I demonstrate that the Lorentz force is balanced by the Coriolis force in the dipolar dynamo regime models resulting in magnetostrophically balanced dynamos whereas the Lorentz force is balanced by the Inertial force (and not the Coriolis force) in the multipolar dynamo regime models resulting in a non-magnetostrophically balanced dynamo. The generation mechanism differs between the regimes and depends on the velocity boundary conditions. The zonal flows of the stress-free models are stronger than in the no-slip models, and bistability is more prominent when stress-free boundary conditions are used. A single scaling law may be feasible for all the models, but there does appear to be some variation for models with different thermal and velocity boundary conditions. The results presented in this thesis are not only applicable to the geodynamo, but will also aid in understanding the dynamos of other planets and exoplanets.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/43543 |
| Date | 08 January 2014 |
| Creators | Dharmaraj, Girija |
| Contributors | Stanley, Sabine |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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