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This thesis addresses the generation and propagation of waves in Earth's fluid outer core, with the objective of better understanding their role in maintaining our planet's magnetic field. The classes of waves studied, which owe their existence both to the rapid rotation of the Earth about its axis and the vibration of magnetic field lines, are united by the fact they are dispersive - which is to say, the velocity at which a given wave travels is a function of the spacing and orientation of its wave crests; the behaviour of such waves is remarkably rich and has frequently counter-intuitive consequences. Our analysis begins at the very smallest scales of the convection which stirs the liquid iron outer core, asking how localised features - such as turbulent eddies or buoyant parcels - might incite oscillatory motions at this fundamental scale. Of particular interest are the mechanisms by which columnar flow structures are established and sustained. We show that, in a situation where the wave source is threaded by a large-scale magnetic field, these processes are managed by a previously-overlooked denomination of oscillations dubbed inertial-Alfvén waves. These objects establish the starting point for an investigation into the journeys embarked upon by packets of waves throughout the outer core, following their progress as global-scale variations in the magnetic field force them to evolve both their spatial structure and propagation velocity. A growing ambient field slows them considerably, even to the point where certain rays are arrested completely and quenched by electromagnetic losses. It is seen that the whole spectrum of small-scale waves influenced by rotation and mean magnetic field have a part to play in this story. Finally, we study a rather distinct variety of waves: hydrodynamic quasi-geostrophic Rossby waves, which rely upon the rapid background rotation and the slope of the core-mantle boundary for their existence. Our interest in these waves is as a possible explanation for the westward drift of the non-dipolar part of the observable magnetic field. As it happens, the crests of these waves invariably progress eastward; however, for a certain subset - corresponding to radially-extended sheet-like flows compatible with the convective driving - they can nevertheless convey energy to the west. This insight raises an intriguing new possibility in the mission to solve this centuries-old puzzle.

Archaeomagnetism is an area of research that utilises the magnetic properties of archaeological materials to date past human activity. This research aimed to use the evidence of past geomagnetism, as recorded by archaeological and geological materials, to identify and characterise short timescale changes in the Earth¿s magnetic field. This contribution to the discipline focused on the first millennium BC, as there is evidence that during this time the Earth¿s magnetic field experienced rapid changes in direction. This work focused on an established weakness in archaeomagnetic studies, i.e. the application of archaeological information to assign a date range to the magnetic directions. The date ranges for 232 magnetic directions from 98 Iron Age sites were reviewed and a programme of fieldwork produced 25 new magnetic directions from 11 Iron Age sites across Britain. The approach developed in this thesis has made significant improvements to the data examined, which represent the prehistoric section of the British secular variation curve (SVC). These data have been incorporated into the British archaeomagnetic dataset that now comprises over 1000 magnetic directions and will be used to generate future British SVCs. The potential of the near continuous records of geomagnetic secular variation from British lake sediment sequences to SVCs was explored. This showed that these sediments have recorded the relative changes in the Earth¿s magnetic field but the dating and method of constructing the British master curve requires revision. As SVCs are predominately used as calibration curves for archaeomagnetic dating, this work provides a foundation for a revised and extended British SVC. This revision would be to the mutual benefit of studies in archaeology and archaeomagnetism, as the latter could potentially enable highresolution dating of Iron Age material, providing a viable alternative to radiocarbon dating.

Earth’s global magnetic field envelops us all, protecting us from cosmic rays, aiding our navigation, and shielding our oxygen-rich atmosphere. Yet details about its origin, operation, and future remain unknown. Recent space-based magnetic observatories let us observe the field more precisely than ever before, and we can use these measurements to study the deep interior of the Earth and illuminate a region of our planet previously observed mainly by listening to the echoes of earthquakes. In this thesis, I use observations of changes in Earth’s magnetic field to study the structure and processes occurring near the top of Earth’s core. In particular, I examine the long-debated question of whether a stratified layer of fluid, termed by some as the stratified ocean of the core or SOC (Braginsky, 1998), lies at the top of Earth’s outer core. I first implement a model to simulate fluid motions in the SOC, which I describe in chapter two. Then, I derive the properties of a class of eastward-propagating equatorially trapped magnetic waves I term eMAC waves in chapter three and develop a statistical threshold to detect these waves in chapter four. In chapter five, I apply my statistical test to observations of Earth’s geomagnetic field and find evidence for these waves in Earth’s core. Finally, in chapter six, I propose a 20-40 km thick SOC with buoyancy strength of N ≥ 10 Ω to support the observed eMAC wave modes. I then discuss the implications of this layer, propose a double-layer SOC as a way to reconcile eMAC signals with previous observations, and enumerate possible future avenues of investigation.