A New Energy Budget for Earth's Core and Implications for the Geomagnetic Field

Earth has possessed a magnetic field for at least the last 3.5 billion years, a fact that has profound implications for the evolution of our planet. The geomagnetic field shields the surface environment and the many orbiting satellites from potentially harmful incoming solar radiation; long ago, this shielding effect facilitated the formation of a breathable atmosphere. The field strength is far from constant, varying from place to place and also in time; indeed, the field strength has been decreasing for the last 150 years, leading to a weakening of our protective shield. On a more regional scale, patches of weak field can develop, such as the current low located in the southern Atlantic, which is known to cause anomalies and even failures in satellites that pass through it and has additionally been linked to the global decrease in geomagnetic field strength and local climate variability. Some predictions suggest that this patch of weak field will grow over the next 100 years, which could have significant consequences given society's increasing reliance on satellites and electronic infrastructure. Elucidating the processes that produce global and regional changes in the magnetic field is fundamental for predicting future behaviour.

The source of Earth's magnetic field lies inside the outer core, a region of molten iron some 2800km below Earth's surface. Magnetic field lines, like strands of spaghetti, emanate from the outer core and thread through the whole Earth, passing through the surface and off into the atmosphere. This field is generated by vigorous motion of the molten iron, which twists and stretches the magnetic field lines, a process that requires a significant amount of energy to maintain. The amount of available energy determines the behaviour of the molten iron (just like the behaviour of water in a heated pan depends on the temperature of the stove), which in turn dictates the strength and structure of the magnetic field. In a significant development, my recent work has shown that the energy available to power the molten iron into motion, and hence generate the magnetic field, is presently 2-3 times smaller than previously thought. This result implies that the behaviour of the molten iron in Earth's core may be very different to current predictions (imagine how the water reacts after turning the stove temperature down from boil to simmer), and that current interpretations of the processes causing our magnetic field to vary in space and change in time may be incorrect. At a more fundamental level, we do not currently know how our planet has managed to support a magnetic field for much of its history because the present-day energy reduction causes significant problems for all previous models that explain the existence of the field for the last 3.5 billion years.

The dramatic reduction in energy available to Earth's outer core is prompting one of the biggest changes to our understanding of the geomagnetic field in the last 20 years. To reestablish a basic theory that explains the long-term existence of the magnetic field requires a model that describes how the outer core has evolved over time and therefore arrived its present-day state. I have recently developed a new mathematical model of outer core evolution that alleviates the technical difficulties encountered by previous models. Over the next five years I will use this model to understand how the Earth has supported its magnetic field for the last 3.5 billion years, thereby providing fundamental new sight into the most remote and enigmatic region of our planet. I will use this information to make computer simulations of the Earth's outer core, which will establish the processes responsible for producing the complex magnetic field behaviour we observe and make predictions about future behaviour of the field including the evolution of the global field strength and patches of weak magnetic field.

The nature of my proposed research means that the greatest impact will be within the Earth Science academic community. My work will tackle fundamental problems with our understanding of the evolution of Earth's deep interior and the dynamical processes generating Earth's magnetic field and I therefore expect the outcomes to have significant impact in the disciplines of core dynamics and geodynamo theory. My new core evolution model will constrain fundamental Earth properties, such as the core-mantle boundary heat-flux, that will be of significant importance to the international community working on mantle dynamics. Additionally, my research will constrain the evolution and properties of the stable layer at the top of Earth's outer core, predictions that can be independently tested using seismological methods. Finally, the new simulations of Earth's core that I will develop engender a two-way link with geomagnetism and paleomagnetism: my simulations will elucidate the processes that give rise to observed spatial and temporal changes in the magnetic field and will also make predictions about past and present behaviour that can be tested with data. I anticipate that establishing close links between observation and theory will provide stimulus to both the future interpretation of paleomagnetic measurements and the direction of theoretical dynamo research.

Results of the proposed research will be presented to both specialists in geodynamo modelling and the broader scientific community through a combination of international conference presentations (as outlined in the accompanying justification of resources--e.g. AGU and SEDI). I expect the findings of this research to have sufficient impact to warrant publication in journals such as Nature and Science as well as more specialised journals such as Earth and Planetary Science Letters, G Cubed, Geophysical Journal International, Physics of the Earth and Planetary Interiors, and the Journal of Fluid Mechanics.

I believe that my proposed research can be of significant interest to the general public. Although fundamental Earth system science does not have the immediate impact of other disciplines such as climate change or sea level rise there has been plenty of recent media attention focused on deep Earth science (specific examples of television programs and popular press articles, including those focusing on my recent work, are given in the accompanying "Pathways to Impact" document), which must reflect public interest and curiosity in the subject. The extent to which the public benefit from my research will be directly related to the size of the audience I can engage and I have outlined several approaches in the accompanying "Pathways to Impact" document that will facilitate accessible communication of my research to the scientifically-curious public. I aim to publish articles on the broader implications of our renewed understanding of Earth's deep interior in the periodicals "New Scientist" and "Physics World", which have a combined circulation of almost 200,000. I will also continue collaboration with the University of Leeds press office and NERC's own scientific writers to maximise the exposure of my work, targeting NERC publications and broadsheet newspapers such as the Times, Observer and Guardian, which each run dedicated science sections and have a combined weekly circulation of almost 1 million. Finally, I will create a website that presents an interactive "Tour of the Deep Earth". The site will allow the user to view components of the deep Earth system at various levels of detail; at each level, information will be provided in the form of text, cartoons, and movies (where appropriate) in a style that is aimed at the non-expert. A discussion forum will allow users to leave comments and suggestions for future improvements. The website will provide a permanent and evolving resource for public engagement that I will popularise through my other outreach activities.