The Earths's Core: Dynamics and Reversals

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Obtaining an understanding of the physical mechanisms responsible for the generation of the Earth's magnetic field is one of today's outstanding scientific challenges. Paleomagnetic records provide a long history of the Earth's field, revealing long epochs in which the magnetic field is of a certain polarity, interspersed with relatively short periods during which the field reverses.Understanding why the Earth's magnetic field exhibits this characteristic behaviour can only come from a full understanding of the processes that maintain the magnetic field against its tendency otherwise to decay -- the geodynamo mechanism. The interior of the Earth, beneath the crust, has three distinct regions: a solid, predominantly iron, inner core; a liquid metal outer core; and an electrically conducting mantle, in which motions can occur only over extremely long time scales. The dynamo is thus located in the outer core, and results from the motions in this region maintaining the magnetic field via induction. The most widely accepted theory for the motions of the outer core is that they result from a combination of thermal and compositional convection.Although the equations governing the dynamics of the Earth's core are known, they cannot be readily solved, owing to the extreme values of the dimensionless parameters involved. However, with today's extremely powerful, parallel processor computers, it is possible to go some way towards the true parameter regime and, crucially, then to obtain new insights into the physics involved, and, subsequently, to lead to new physical explanations.We therefore propose to investigate, via numerical simulations on massively parallel computers, dynamo action driven by rotating thermal convection. Previous studies of this problem have revealed that in certain parameter regimes the magnetic field is small-scale, and hence not reminiscent of the Earth's dipolar field, whereas if the rotation rate is sufficiently rapid then the convection is organised into coherent columns, and these can generate a strong large-scale magnetic field. It has been conjectured that dynamos such as the Earth's, that maintain one polarity for a long period but also undergo intermittent reversals, lie on the boundary between these small- and large-scale dynamos. Currently little is known about the nature of the transition between these two types of dynamo. Our first aim is to understand this transition in a plane-layer geometry, which is computationally very efficient and will allow a thorough exploration of the three-dimensional parameter space governing the problem. Then, with the knowledge afforded by the plane layer problem, we shall conduct a series of focused computations in the more realistic, but computationally more demanding, spherical shell geometry.One of the crucial aspects of any dynamo calculation concerns the nature of the imposed boundary conditions -- on the temperature, the velocity and the magnetic field. In the Earth itself these are complex, and it is therefore very important to understand the implications of the various conditions. For example, will a slowly changing heat flux affect the nature of the dynamo mechanism and maybe the pattern of reversals?Finally, with the considerable computational power now available, we hope to be able to perform sufficiently long runs so as to produce statistics of reversals, thus allowing a direct comparison with the true statistics of the Earth's magnetic field.