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Observations and characterisation of rapid variations in the Earth’s internal magnetic field

Brown, William James (2015) Observations and characterisation of rapid variations in the Earth’s internal magnetic field. PhD thesis, University of Leeds.

Text (William James Brown; University of Leeds thesis; Observations and characterisation of rapid variations in the Earth's internal magnetic field; 2015)
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The internal magnetic field of Earth is generated by dynamo processes in the fluid outer core, variations in flow resulting in the constantly changing form of the magnetic field. The rapid dynamics of the core are largely unknown; the mantle and crust filter and mask small-scale spatial and temporal features and field sources external to the Earth contaminate the limited observations. Geomagnetic jerks represent the most rapid observed variations of the internal field, on the scale of months, and are poorly understood. Jerks are sharp changes between periods of an otherwise linear rate of change of the field. The main aims of this thesis are: to systematically catalogue observations of jerks, focussing on 1957–2008, and to quantify their characteristics; to assess how representative observations of jerks are through synthetic field modelling; and to evaluate what the results of these analyses inform us about the nature of the source of jerks in the core. I identify jerks in observations via a two-step method, removing contaminating external signal before identifying possible times at which a simple jerk model produces a good fit to data. I quantify the properties of the model and fit, providing uncertainty estimates, all with minimal prior information on the occurrences of jerks. Jerks are found to be frequent, regional and not globally contemporaneous. Jerks are identified every year but with relative abundance in 1968–71, 1973–74, 1977–79, 1983–85, 1989–93, 1995–98 and 2002–03. The amplitudes of jerks in Europe are seen to vary periodically, suggesting a regular generation mechanism. Limited observations preclude a global assessment of this. I create stochastic synthetic field models to assess the significance of observations and to infer compatible jerk characteristics. These models reinforce the view that jerks occur in localised patches at the Earth's surface without a consistent temporal or spatial distribution. Spherical harmonic (SH) analysis of the synthetic and observed jerks suggests that despite the localised nature of jerk signals, they can be represented by a potential field with most power below SH degree 4. A lag time of 6 months between variations in Earth's rotation and geomagnetic jerks suggests that sufficiently electrically conductive material must be present in the lower ~130 km of the mantle. At the core-mantle boundary, the power spectrum of the SH jerk models and the demonstrated ability to model jerks with a stochastic process, indicates a chaotic, turbulent core-surface flow regime is a likely generation mechanism.

Item Type: Thesis (PhD)
Keywords: Geomagnetism; Geomagnetic jerk; Secular variation; External field; Stochastic modelling
Academic Units: The University of Leeds > Faculty of Environment (Leeds)
The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds)
The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute of Geophysics and Tectonics (Leeds)
Identification Number/EthosID: uk.bl.ethos.667698
Depositing User: Mr William James Brown
Date Deposited: 13 Oct 2015 13:49
Last Modified: 25 Jul 2018 09:51
URI: http://etheses.whiterose.ac.uk/id/eprint/10392

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