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Title: Core flow modelling : constraints from dynamo theory
Author: Horncastle, Edward Thomas
ISNI:       0000 0004 2670 951X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2008
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In recent history our understanding of the magnetic field and the generating motions of the molten iron in the Earths core have increased dramatically. The two major approaches to investigate fluid flow have been core surface flow modelling from magnetic data inversion and modelling of the dynamo itself. Core flow modelling involves downward continuation of the magnetic field to the core mantle boundary (CMB), then adopting the frozen flux approximation plus added assumptions e.g. tangential geostrophy to reduce non-uniqueness, to obtain fluid flow at the surface of the core that produces the observed secular variation (SV). The main check on the validity of these flows has been observed changes in length· of day. This study aims to test the fluid flow inversion more rigorously by using synthetic data of main field, SV and fluid flow from two self-consistent convection driven dipole dominated dynamos. The dynamo magnetic data are inverted and comparisons made with the true dynamo flow. The use of two large scale assumptions, the strong norm and the KE norm, has been tested. Forward models of advection, a neglected advection, and diffusion, from the dynamo data have been calculated to compare contributions to the secular variation. It is shown that within the dynamos the definition of the magnetic Reynolds number is flawed, relating to a failure of the frozen flux approximation. The effects of truncation of field and flow on the generated advection has been studied. It was found that both the failure of the frozen flux approximation and truncation had a large effect on the flow inversions. Another possible reason for non-recovery of some parts of the flow was found to be that much of the true and inverted flow was along contours of Br / cos 0, the null space caused by the geostrophic assumption. With reducing this non-uniqueness in mind, the validity of of a new assumption called helical flow was checked by studying the true properties of the dynamo flow. A new spectral helical flow constraint that can be applied separately to tangential geostrophy has been developed. With the caveat that the results have been found on dynamos with parameters very different to the Earth, cautious conclusions have been made on the best combinations of assumptions to use in Earth core flow models. It has been shown that, at the truncation of Earth models, when the new helical flow constraint is used with the KE norm and weak geostrophy more of the dynamo true flow has been recovered. The results have been applied to the Earth and validated by using changes in the length of day.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral