The Earths's Core: Dynamics and Reversals

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.

The Earth's magnetic field, how it is generated, the fact that it reverses, and the conditions in the Earth's core are issues that interest the general public. The applicants have been involved in a number of outreach activities over the last ten years, including giving radio and TV interviews on the magnetic fields of the Earth and other planets, and giving lectures to local UK scientific groups. We have also been going into UK Schools to talk to students about our scientific work, and have had involvement with 'Science week' projects. We believe this has a positive effect in getting the public in general, and children in particular, interested in science, and so we propose to devote time to these activities throughout the proposed grant period. The PI, co-I and PDRA would expect to give at least six talks between them to general non-specialist audiences over the grant period.

We are also regularly contacted by the scientific press, e.g. Physics World and Physics Today, for our views on recent progress in our understanding of the Earth's deep interior. We expect this activity to continue for the duration of this project.

Our understanding of processes in the Earth's deep interior is developing very rapidly, and the public is interested in these new developments. Under NERC funding obtained on a previous grant, a website will be developed, devoted to 'Earth's Deep Interior', which we believe will be attractive to the scientifically curious. We shall contribute ideas to this site, providing discussions not only of our own research but also of the science of magnetic reversals in a much wider context, including their possible terrestrial implications. The PDRA will be sent on a media training course, to gain familiarity with promoting science in general, and geophysics in particular, to the public.

The proposers have experience in running RAS discussion meetings, which are an excellent way of disseminating research to a wide audience of scientists, and they would propose to organise a meeting on Geomagnetic Reversals to be held in London during 2014.