VIRGIL: The VIRtual paleomaGnetIc Laboratory

Earth's magnetic field has been part of our planet from early on its formation, but the processes by which it occurs are still not well understood. The first theory of magnetism by William Gilbert proposed one of the first theories of geomagnetism in his treatise "De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth)" in 1600. By observing lodestone (naturally occurring magnetite deposits), Gilbert proposed that the Earth was a giant dipolar magnet, but the theory didn't account for the many small-scale variations in Earth's field over the globe such as the south Atlantic magnetic anomaly. Modern theories of geomagnetism propose that the field is formed by a vast ocean of liquid iron swirling around a solid iron core - and evidence of this exists in modern day observations the fine structure of the field.

But what about the field in the past? Fortunately, rocks containing small specs of magnetic materials can record and retain Earth's ancient field over millennia, in the same way that magnetic tape can be used to record information. Unfortunately, like magnetic tape left in the sun too long, natural rocks are subject to many external factors that distort the ancient signal that they record. It is the task of palaeo- and rock magnetism to devise experiments to "clean" the signal and recover the ancient field. But how do we know that the field that we recover is the true ancient recording? This is done by understanding precisely how each small grain behaves in response to heating, changing external field and other processes (such as chemical alteration).

Knowledge of Earth's field tells not just the story of the physical processes of how our planet was formed - it is also the story of human culture. It constrains dates of archaeological sites and gives us an understanding of how those sites were used, be it for cooking or the forging of metals. It tells us of natural disasters such as wildfires and the displacement human populations. The ancient field has always been with us and or planet, but the physical theories that underpin experiments to recover this vital information is flawed.

The current theory of magnetic recording in natural materials was devised by Néel and Stoner & Wohlfarth and is now 70 years old. It accounts for only a tiny fraction of the grain sizes and geometries of magnetic minerals found in rocks; but the experimental protocols and analytical tools to recover the palaeomagnetic signal are still being built on this model. In this proposal I will overhaul magnetic recording theory by using micromagnetic modelling, big-data and machine learning methods to build a complete simulation of the recording process in rocks from the ground up. This will enable me to revisit the palaeomagnetic record and answer the question: "is this a good recorder"? Such an evaluation of the existing palaeomagnetic record is critical since it constrains geodynamo models and allows us to peer into the deep past of our planet. Not only that, but it will also help underpin the models that will allow us to investigate the future of the geodynamo.