A New Generation of Forward and Inverse Geodynamo Models

Have you ever wondered why compasses point north? This fundamental question, still unsolved, has perplexed many scientists including the likes of Einstein. Scientists now accept that the source of the geomagnetic field lies inside Earth's core and, like a giant bar magnet thousands of miles beneath our feet, radiates tentacle-like magnetic field lines that penetrate the mantle and wrap around the Earth. Although it is apparent that, at the Earth's surface, the geomagnetic field is principally dipolar (having north and south poles), images of the field at the edge of the core show many more complex features. Furthermore, the geomagnetic field varies slowly with time. Although over the duration of a human lifetime these changes are small, geologists have shown that, over the course of the Earth's history, the geomagnetic field globally reverses: the north and south poles swap places a few times every million years or so. The geomagnetic field is not only crucial for navigation (by many animals, as well as humans) but provides us with an electromagnetic shield that protects our planet from harmful solar and interstellar radiation, and consequently has a potentially important influence on climate. Of particular interest is the recent evidence that the Earth's dipolar field is losing strength and that it may completely reverse within 2000 years. During reversals the electromagnetic shield is significantly weakened and, although our ancestors have survived many such events apparently without harm, it is unknown how the many satellites and other technological infrastructure, that our society relies upon, will be affected. A full understanding of the generation of the Earth's magnetic field and its reversals would undoubtedly help us to limit any destructive effects that might occur. The geomagnetic field is generated by the so-called geodynamo mechanism, powered by motions of molten iron in the Earth's core. Scientists study the geodynamo by using complex computer models that simulate the processes in the core. However, because of severe computational difficulties inherent in current techniques, the physical properties of the modelled Earth must be altered in order to make the problem solvable on present-day computers. It is not possible to faithfully simulate the Earth's core, and opinion remains divided over the many interpretations of such geodynamo models, including the trigger for global reversals. An additional technique is to use observations of the magnetic field on Earth's surface to image the field and the structure of the flow of molten iron at the edge of the core. This has been enormously successful in revealing many interesting dynamics although, rather like attempting to understand the internal currents of the world's oceans by simply inspecting the ocean surface, it is impossible to say very much about the processes inside the core. Consequently, many aspects of the Earth's interior and source of the geomagnetic field remain a mystery. Earlier this year in an exciting new development, I presented some crucial theoretical advances that pave the way for constructing a new generation of geodynamo models. The new technique supplies a fundamentally new approach that allows realistic properties of the core to be accessed directly on computers, and I will be able to produce the first geodynamo models able to simulate the Earth's core accurately. This will enable robust characterisation of the mechanism that generates the geomagnetic field and detailed analysis of the associated phenomena, including magnetic reversals. Additionally, by further exploitation of the new theoretical developments, I will be able to use observations to probe the present-day magnetic field structure inside the core, again something that has never before been possible. These two complementary projects will revolutionise the state-of-the-art in geodynamo modelling and will provide unprecedented insights into the dynamics of the Earth's core.