A new model of the geodynamo: large-scale vortices in the Earth's core

For centuries, humans have been aware of the presence of a magnetic field on Earth because of its action on magnetised objects, such as the needle of a compass. Scientific instruments that measure its strength and direction show that the Earth's magnetic field (called the geomagnetic field) is predominantly dipolar at the Earth's surface, like the magnetic field produced by a bar magnet. The instruments further reveal that the geomagnetic field displays more complex features, such as regional patches (of about 1000km in radius) of reversed polarity. The geomagnetic field varies slowly on a human lifetime, but over the course of the Earth's history, geophysicists have shown that it varies considerably and sometimes undergoes global polarity reversals, where the north and south magnetic poles swap places. These global reversals occur a few times every million years or so. Each global reversal takes only about 5000 years, and during this time, the geomagnetic field is weak and probably disorganised. The geomagnetic field is not only crucial for navigation (used by many animals, as well as humans) but provides us with an electromagnetic shield that protects our planet from harmful solar radiation. During a global reversal, this electromagnetic shield is significantly weakened, and if a reversal occurred today it would cause tremendous damage to space satellites and electrical power grids. The last global reversal occurred about 780,000 years ago, long before the advent of our modern technologies. The magnetic field strength has been decreasing for the last 150 years, coinciding with the appearance of the regional patches of reversed polarity. Whether these reversed patches are precursors for a global reversal is unknown, as is the cause of the global reversals. To predict the changes in the geomagnetic field, which would help us limit potential destructive effects, we need to better understand the processes that generate the geomagnetic field.

The geomagnetic field is generated deep inside the Earth, in the outer core, which is composed of molten iron. Motions of molten iron generate electric currents that induce the magnetic field, through a physical process called geodynamo. The geodynamo is governed by nonlinear mathematical equations, which can only be solved with the help of computers. However, even the most powerful computers struggle to model the extreme conditions that prevail in the core and its exact physical properties. Thus the computer models use strongly altered physical properties in order to make the problem solvable on present-day computers, potentially leading to inconsistencies when rescaling the results of the models to the core properties. In particular, current models find that the geodynamo is produced by motions of molten iron of only about 100m. However, theoretical arguments about the generation of the geomagnetic field imply that these motions occur on much larger spatial scales, and this conclusion is reinforced by the observation of the patches of reversed polarity that measure about 1000km across. A key mechanism is therefore missing in the current models to explain the formation of large-scale fluid motions.

During my fellowship, I will address this problem by studying a new mechanism that explains how large-scale flows can form under the conditions that prevail in the Earth's core. This new mechanism is based on my recent work using a simplified computer model: I demonstrated the formation of large-scale, long-lived cyclones (somewhat similar to the tropical cyclones observed in the atmosphere) from turbulent smaller scale motions. I will study whether these cyclones can be present in the Earth's core by extending my previous results to a realistic model of the core, and whether they can produce Earth-like magnetic fields. I will then investigate whether the patches of reversed polarity are associated with these large-scale cyclones and whether they are precursors for the global reversals.

1) Increase the reliability and accuracy of long-term space weather predictions
The proposed work will provide a better model of magnetic field generation in the Earth's outer core, which is needed to explain global polarity reversals and the evolution of regional patches of reversed polarity. In particular, the fellowship aims to provide a timescale for the weakening of the magnetic field prior to and during global polarity reversals, and to predict the strength, spatial distribution and temporal fluctuations of the geomagnetic field. These three factors are crucial to space weather predictions, which forecast the electromagnetic conditions in near-Earth space with important impacts on modern technologies: satellites, electricity power grids, radio and telephone communications, and geophysical exploration. Space weather is governed by complex interactions between the geomagnetic field and the solar wind. An example of the economic impact of space weather has been quantified in a recent study that found that power surges caused by space weather account for 500 industrial insurance claims in North America annually (Schrijver et al., 2014, doi:10.1002/2014SW001066). A famous example of societal impact is the widespread electricity blackout for over nine hours that affected 6 million people in Canada in 1989 due to a geomagnetic storm caused by a massive burst of solar wind.
Currently, our ability to forecast space weather is limited because of our uncertainty in both short-term and long-term trends in the Earth's magnetic field. The results of this fellowship could therefore improve long-term space weather predictions (i.e. on secular timescales), and potentially mitigate the damage caused by regional and global weakening of the geomagnetic field. The potential beneficiaries are the power industry, global communication industry, geophysical exploration companies that conduct magnetic surveys, and astronauts on manned space flights who are at risk of exposure to cosmic and solar radiation. The potential beneficiaries will be reached through research offices working on space weather predictions, and in particular, the British Geological Survey, which is interested in long-term changes in the geomagnetic field. In the UK, the British Geological Survey has undertaken studies into the geomagnetic hazard, for example, for the National Grid Company of England and Wales and Scottish Power.

2) Public engagement
After the recent presentation of the first magnetic data collected from the SWARM satellite mission launched in Nov 2013 and sponsored by the European Space Agency, a number of newspapers and science blogs reported on the information that the measurements confirm the global weakening of the geomagnetic field, with the most dramatic declines over the Western Hemisphere (e.g. Daily Mail, Scientific American, Universe Today). Journalists immediately linked this observation with the possibility that the geomagnetic field might reversed its polarity soon and speculated on the impacts of such an event on our modern society, sometimes using apocalyptic headlines (e.g. in the MailOnline 31/01/2014, "Forget global warming, worry about the magnetosphere"). However, whether this global decrease of the magnetic field strength is linked with an upcoming global polarity reversal is unknown, as is the strength and spatial distribution of the magnetic field during the reversals. The work proposed during my fellowship aims to elucidate this issue. Since this issue has already attracted the interest of the media, there are definite impacts of the outcome of my research on public awareness.
Rather than simply developing my own scientific website, which would likely only reach a limited viewership, I will engage with popular scientific blog/website editors and with national newspapers through the media offices of the Natural Environment Research Council and the University of Leeds.