How Earth’s Magnetic Field Influences Flows in the Planet’s Core
(This article has been published long ago, just re-posted in this blog for further reference. Anyhow, comments are welcome.)
October 31, 2024• Physics 17, 142
A “Little Earth Experiment” inside a giant magnet sheds light on
so-far-unexplained flow patterns in Earth’s interior.
Sketch of outer-core flows derived from data from the Swarm mission (plotted looking down on the North Pole). The black lines indicate flows averaged over two decades, showing patterns that violate the Taylor-Proudman theorem by crossing a boundary defined by the solid-core radius. The red and blue colors indicate azimuthal-flow components.
Earth’s inner core is a hot, solid ball—about 20% of Earth’s
radius—made of an iron alloy. The planet’s outer core, beneath the rocky
mantle, is a colder, liquid metal. Geophysics models explain that, since the
movement of a liquid metal induces electrical currents, and currents induce a
magnetic field, convection and rotation produce our planet’s magnetic fields.
But these models typically neglect an important contribution: how Earth’s
magnetic field influences the very flows that generate it. Alban Pothérat of
Coventry University, UK, and collaborators have now developed a theory that
accounts for such feedback and vetted it using a lab-based “Little Earth
Experiment” [1]. Their results
inform a model pinpointing processes that might explain the discrepancies
between theoretical predictions and satellite observations of Earth, opening
new perspectives on the study of geophysical flows.
These observations can’t be explained by the established theory for rotating fluids, which assumes that magnetic-field-induced forces on the flow can be neglected, as they are dominated by rotation-induced Coriolis forces. If rotation is fast enough, goes the theory, flows of liquid are two dimensional and lie in the plane normal to the rotation axis. For planetary interiors, this imposes a constraint known as the Taylor-Proudman theorem: Fluid cannot flow across a boundary, called the tangent cylinder, defined by a radial distance equal to the solid-core radius. The flows documented on Earth, however, violate this condition. To explain the discrepancy, what’s needed is an experiment that captures convection, rotation, and magnetism all at once, says Pothérat.
Sketch of the setup
used by Pothérat and co-workers. A liquid-containing hemisphere, a heating
element, and a water tank represent Earth’s outer core, inner core, and mantle,
respectively. The structure sits on a rotating table in a 10-tesla magnetic
field, while a laser injected from the top allows the researchers to map the
flow field.
Pothérat and his colleagues have realized such an experiment,
using it to vet an extension of the established model associated with the
Taylor-Proudman constraint. Previous Earth-mimicking setups used rotating tanks
filled with a highly conductive but opaque liquid metal, which prevented the
visualization of liquid flows. The team’s “Little Earth Experiment”
(Fig. 2) instead used a
low-conductivity, transparent sulphuric acid solution, which allowed the
acquisition of maps that visualize flow structures in a magnetorotating fluid.
“That’s the big originality,” says Pothérat. Since their liquid was much less
conducting than a liquid metal, the team needed a large external magnetic field
to create magnetic forces sufficiently strong to mimic planet-like conditions.
They achieved such conditions using a 10-tesla magnetic field produced by the
giant magnet of the Grenoble High Magnetic Field Laboratory in France. “The
real feat of our experiment is fitting a rotating tank inside this enormous
magnet,” says Pothérat.
The Little Earth Experiment involves a rotating hemispheric dome
sitting on a flat, rotating table. The transparent-glass hemisphere was filled
with a conductive fluid, which represented the outer core of Earth. At the
hemisphere’s flat bottom, a cylindrical heating element protruding into the
liquid played the role of Earth’s inner solid core, driving convection. A water-filled,
cylindrical plastic tank sat on top of the hemisphere, providing a cooling
effect that mimicked that due to Earth’s mantle. By lacing the conductive fluid
with hundreds of thousands of micron-diameter hollow glass particles, which
scattered incoming laser light, the researchers could track the particles’
positions and measure the fluid’s velocity at multiple points as the tank
rotated.
The key measurements were maps of the fluid’s velocity at two
heights—one near the solid core and one at latitudes close to the top of the
hemisphere—obtained for electromagnetic and rotational forces representative of
Earth-like conditions. Fitting such data, the researchers calculated that at
least 10% of the liquid flowed in a circulating pattern similar to that occurring
in Earth’s core: From the solid core the liquid flowed toward the top of the
dome and then toward the equatorial regions. “We can finally see what the flow
looks like,” says Pothérat.
The researchers established that the polar component of the magnetic
field drives flow between polar and equatorial regions, concluding that this
field-induced flow must be taken into account for explaining convective flows
that can’t be described by rotation alone. Modifying the conventional theory by
adding a magnetic field pointed in the polar direction allowed the researchers
to predict exactly when and how much liquid flowed across. The magnetic field
fully explains why the flow crosses the tangent-cylinder boundary, says
Pothérat.
Hao Cao, a geophysics researcher at the University of
California, Los Angeles’s Department of Earth, Planetary, and Space Sciences,
calls the work “impressive” and says that the experimental verification of flow
regimes “illustrates the critical role of magnetic fields in shaping fluid dynamics
in Earth’s and planetary cores.” He adds a cautionary note regarding its direct
application to real planetary cores, pointing out that “the fluid dynamics in this experiment has minimal impact on the
magnetic field itself.” That’s not the situation expected in planetary cores,
he says.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- A. Pothérat et al.,
“Magnetic Taylor-Proudman constraint explains flows into the tangent
cylinder,” Phys. Rev.
Lett. 133, 184101 (2024).
