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

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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.