A Multidisciplinary Study of Thermal Core-Mantle Coupling in Geodynamo Models

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Earth's magnetic field is about as old as the Earth itself and its presence has broad implications for life on our planet. Used as a navigational aid by humans and animals for centuries, the magnetic field also shields Earth's surface from potentially harmful incoming radiation and protects the many man-made satellites orbiting the planet. Earth's magnetic field is generated in the outer core, a region of molten iron some 2800km below Earth's surface that is sandwiched between the solid inner core and mantle. Magnetic field lines, like strands of spaghetti, emerge from the outer core and thread through the solid mantle, finally appearing at Earth's surface and extending off into the atmosphere. When viewed at Earth's surface these field lines form a dipole, having north and south poles near the geographic poles. When viewed at the core-mantle boundary, the interface between the liquid core and the mantle, a much more complex picture emerges: the field contains four regions where magnetic field lines clump together, creating a patch. The patches are located under Canada, south America, Siberia and Australia. Earth's magnetic field also exhibits variations in time. Patches are stationary over the past 400 years, coinciding with the time-span over which the magnetic field has been directly measured, but appear to be more mobile over longer time periods. The most dramatic events are geomagnetic reversals, where the north and south magnetic poles flip. During reversals the shielding effect of the field is weakened and the impact on orbiting satellites could be severe; the effect of a reversal is unknown but life has survived the many reversals that have occurred to date. A complete understanding of how the Earth's magnetic field is generated and how reversals occur is of paramount importance for mitigating against the potential effects of such events. Crucial to this challenge is establishing why the present day field appears as it does, i.e. why is contains patches, whether patches are permanent features of the field, and whether there is a link between the present field and the field during and after a reversal. We model the physical processes in the outer core using computer simulations. Unfortunately, the task of simulating conditions in Earth's core is too great for current computers and so simplifications to the physical properties of the outer core are required in order to make progress. I have shown that simplified models produce realistic magnetic fields, including patches, when the flow of heat from the core to the mantle varies across the core-mantle boundary. This is the situation that occurs in the Earth: heat passes from the outer core to the mantle because the Earth is hottest in the middle, but more heat is lost beneath Africa and the Pacific than under Australia and the Gulf of Mexico. While predictions from these simple models are encouraging, the mechanism by which the patches are maintained is unlikely to apply when the physical properties of the real Earth are used in simulations. Moreover, the simulated fields do not contain the variability (such as reversals) seen in the observations. Earlier this year I observed a new process that can sustain patches through variations in core-mantle heat flow. The new process is theoretically applicable to the real Earth and is capable of producing the time-varying behaviour seen in observations. Additionally, I have recently presented the first detailed description of the computing power required to simulate physical properties approaching those of Earth's core. In this research proposal I will exploit these two discoveries to produce the first realistic models of the generation mechanism for Earth's magnetic field and elucidate the complex processes occurring in Earth's outer core in a manner never before possible.