The core, comprising the innermost parts of the Earth, is one of the most dynamic regions of our planet. The inner core is solid, surrounded by a liquid iron alloy. Inner core solidification combined with motions in the fluid outer core drive the geodynamo which generates Earth's magnetic field. Solidification of the inner core also supplies some of the heat that drives mantle convection and subsequently plate tectonics at the surface of the Earth. The thermal and compositional structure of the inner core is thus key to understanding the inner workings of our planet. No direct samples can be taken of the core and our knowledge of the thermal and compositional state of the Earth's outer and inner core relies on seismology. Ray theoretical studies using short period body waves are the most commonly used seismological data; these have led to observations of a large range of anomalous structures in the Earth's inner core, including anistropy, layers and hemispherical variations. However, due to uneven station and earthquake distribution, the robustness and global distribution of these features is still controversial. Long period seismic free oscillations, on the other hand, are able to provide global constraints, but lack of appropriate theory has prevented more complicated structures from being studied using normal modes. Thus, many fundamental questions regarding the thermal history of the core and geodynamo remain unanswered. Here, I propose to develop a comprehensive seismic inner core model, employing fully-coupled normal mode theory for the first time and using data from large earthquakes such as the Sumatra-Andaman event of 26 December 2006. This will dramatically change our current ideas of structure in the inner core. Using a novel combination of fluid dynamics and mineral physics I will interpret the thermal and compositional structure found at the centre of our planet, which in turn are fundamental to understand its geodynamo and magnetic field.
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