Understanding Earth’s heart
Earth’s magnetic field shields us from cosmic radiation carried by the solar winds. Kathy Whaler has devoted much of her career to deciphering the complex core flows that sustain our planet’s barrier
Kathy’s undergraduate studies were focused on mathematical physics. While she loved all aspects of physics (though claims to have been useless in the lab), the majority of PhD opportunities centred on enormous facilities, such as CERN in Switzerland, which didn’t hold much appeal. Having studied geography (as well as physics and maths) at A level, Kathy made the move into geophysics and so began her career researching the inner workings of our planet.
The core paradox
Kathy’s PhD research at the University of Cambridge, UK, in the late 1970s addressed the core paradox – the idea that the top part of Earth’s liquid outer core might be stably stratified, potentially reducing the volume of the outer core responsible for Earth’s dynamo and the generation of the magnetic field. Kathy came up with an innovative method to test whether a stratified layer existed at the top of the outer core. Her approach relied on the assumption that if convection doesn’t reach all the way to the boundary with the overlying mantle, then the pattern of flow at the core’s surface should be simple, with no upwelling or downwelling and no horizontal divergence or convergence of flows. And, if so, this simplicity would be detectable in magnetic data.
“I realised that although there is severe ambiguity in determining core surface flow from magnetic models, we can test whether the data are consistent with the surface flow being purely toroidal, comprising gyres, eddies and bulk rotations, in contrast to poloidal, which has upwellings, downwellings, and overturning motion.”
Kathy showed that the then-available models of the magnetic field appeared consistent with simple, purely toroidal flow, implying that a stably stratified layer likely does exist at the top of the core, and that the overturning motions associated with Earth’s dynamo are confined to the deeper levels of the outer core.
She explains that such a stably stratified layer might result from the accumulation of light elements, or the temperature profile dropping below the adiabat. Subsequently other researchers set about further testing whether or not such a layer exists, using various approaches including seismology, geodesy, high temperature-pressure lab experiments, ab initio calculations and computer simulations of the dynamo.
“I’d like to think I set a trend here – subsequently, others proposed different constraints on the flow, which could also be tested against the magnetic data and, if consistent, reduced the ambiguity in our models of the core surface flow.”
Kathy’s later work, based on magnetic field observations rather than models of the data that were not designed for extrapolation to the core surface, shows that a small component of poloidal flow likely does exist in the outer core. However, she notes that this more complex flow makes up only a few percent of the total energy of the core’s flow. It’s also possible that the stably stratified layer doesn’t wrap around the entirety of Earth’s core – there might be some regional variations.
I’d like to think I set a trend here– subsequently, others proposed different constraints on the flow, which could also be tested against the magnetic data
The complex workings of our planet’s heart are not simple to unravel. However, over the past few decades, increased computing power and novel methods and data have provided new means for understanding core flow patterns associated with the geodynamo, including numerical simulations that allow production of more ‘Earth-like’ models of the magnetic field and an increasingly sophisticated understanding of the spatial variations of Earth’s heat flow.
“When I began research, simple one-dimensional models of heat flow had Earth’s cooling driving core convection and powering the dynamo, which expelled heat into the mantle to drive mantle convection. Now we believe that three-dimensional mantle convection patterns mean there are probably places where heat is actually entering the core from the mantle, even though averaged over the core surface heat is leaving; these three-dimensional patterns imprint on the core from the mantle and affect how the dynamo works.”
In the 1970s and 1980s, magnetic data came from a global network of about 150 geomagnetic observatories, most in the northern hemisphere and very few in ocean regions.
“While they were adequate for modelling the field at Earth’s surface, their sparseness meant we could have little confidence in their reliability when extrapolating to the core surface. However, just as I was finishing my PhD, NASA launched the Magsat Low-Earth Orbiting (LEO) satellite mission, which provided global coverage of the field for seven months – long enough to get a reliable snapshot of the core surface magnetic field. Subsequent LEO missions have provided better and longer coverage, enabling determination of how the field at the core surface is changing with time.”
A revolution in this field came with the launch a decade ago of ESA’s Swarm mission, a trio of LEO satellites that allow much better separation between magnetic signals stemming from Earth’s core, the crust, ocean, ionosphere and magnetosphere. The resulting high-quality information on spatial and temporal variations of the core field help constrain models of Earth’s dynamo and call for adjustments to our understanding of core flows.
“Theory tells us that the ‘bar magnet’ field that we observe as the predominant component of Earth’s magnetic field is generated by an equatorially symmetric flow in the outer core. But recently observed changes in Earth’s magnetic field cannot be modelled that way, so I’m currently probing where and how equatorial symmetry is broken.”
Rifting in Afar
Aside from her research on the deep workings of our planet, Kathy also applies her expertise in electromagnetism to understand processes operating much closer to Earth’s surface, by using magnetotellurics – a geophysical method that uses variations in Earth’s magnetic and electric fields to determine electrical resistivity in the subsurface, which can be used to identify pockets of magma. For example, as a participant in the Afar Rift Consortium, which was funded by NERC after a single dyke intrusion produced 8 m of lateral extension over just ten days, Kathy and the multidisciplinary team were able to unravel some of the processes operating during late-stage continental rifting in Afar, Ethiopia.
“We identified copious quantities of melt in places it shouldn’t be – ponding at the top of the mantle, and not directly beneath the rift axis.”
Given the buoyancy of magma, such large quantities of melt are expected to intrude the crust rather than sit at the top of the mantle. This remarkable finding from Afar, together with observations from the main Ethiopian rift of seismicity occurring in unexpected locations, well below the brittle-ductile transition, supports the theory of magma-assisted rifting. In this way, “you can break the continent much more easily”, says Kathy.
Kathy identifies several open questions that research continues to tackle. For example, in the deep Earth, we still don’t know for sure whether a stably stratified layer exists at the top of the core or what light elements might exist in the core. At Earth’s surface, we don’t yet fully understand the details of how continents rift to form new ocean basins, and Kathy explains that it is currently not possible to make forecasts of earthquakes and volcanism in poorly monitored rift zones that are useful for disaster preparedness.
Work in the UK to address such questions has been hampered by the challenges of Brexit. Kathy laments the loss of funding opportunities in conjunction with EU colleagues, as well as difficulties in getting students and employing staff from EU countries.“We are no longer ‘trusted partners’ for collaboration. Our closed borders have created significant fieldwork difficulties for both research and teaching.”
Despite these challenges, Kathy’s work, together with that of her multidisciplinary partners, is helping to transform our understanding of the inner workings of our planet from core to crust to magnetosphere.
Kathryn Whaler OBE is a professor of geophysics at the University of Edinburgh, School of GeoSciences, UK, and the 2023 recipient of the Society’s Wollaston Medal.
Interview by Amy Whitchurch