Scientists have uncovered a previously unknown doughnut-shaped region within Earth's outer core, offering new insights into the dynamics of the planet's magnetic field.
Located thousands of kilometers beneath the Earth's surface, this doughnut-shaped region lies within the liquid outer core, parallel to the equator and confined to low latitudes.
More than a mere quirk in our planet's internal structure, the doughnut's composition could enhance our understanding of Earth's magnetic field—the shield wrapped around us that protects life on the surface from damaging solar winds and radiation.
The vigorous movement of liquid iron and nickel is what forms the magnetic field, a process driven by temperature differences and, crucially, the presence of light elements such as those in the doughnut.
"The outer core is a bit bigger than the planet Mars, yet we know more about the red planet's surface than the core's interior," study co-author Hrvoje Tkalčić told Newsweek. His team's findings, published in the journal Science Advances, have added a giant piece to the puzzle that until now lay undetected.
The Earth itself is composed of two core layers: a solid inner core and a liquid outer core, which is surrounded by the mantle. The newly discovered structure is situated at the top of the outer core, where it meets the mantle.
"The region sits parallel to the equatorial plane, is confined to the low latitudes and has a doughnut shape," Tkalčić said in a statement. "We don't know the exact thickness of the doughnut, but we inferred that it reaches a few hundred kilometers beneath the core-mantle boundary."
The discovery was made possible by a novel approach to seismic wave analysis.
"Like medical doctors who use ultrasound or X-rays, global seismologists can use the waveforms recorded on seismographs worldwide due to the passage of seismic waves after large earthquakes, explosions, impacts and other natural phenomena," Tkalčić said.
"We can use their arrival times, amplitudes or waveform shapes. Understanding how these waves move through the Earth, spreading around, penetrating through, or bouncing off internal boundaries and inhomogeneities is the key."
Rather than relying on traditional methods that focus on signals in the first hour following a seismic event, the scientists analyzed waveforms many hours after the earthquakes. This method allowed them to better measure the internal properties of the core since the waves have time to bounce off boundary structures like echoes in a cave.
"Of course, those signals are tiny as their energy weakens during multiple passages through the core, but we don't really look directly at the weak signals. We detect them by measuring their similarity on many recorders around the globe. The similarity of two weak signals becomes a more significant piece of information than the signals themselves," Tkalčić explained.
The speed at which the seismic waves travel through this doughnut is slower than other regions, the team found, implying a higher concentration of light chemical elements than elsewhere.
Tkalčić added: "Light chemical elements are an essential ingredient driving vigorous convection in the outer core due to their buoyancy, and in turn, that process, paired with Earth's rotation, sustains a geodynamo in the liquid core——the source of the Earth's magnetic field.
"Understanding the spatial distribution of light elements is an essential initial condition for numerical simulations of the geodynamo and understanding the change of its intensity and direction with time."
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References
Ma, X., & Tkalčić, H. (2024) Seismic low-velocity equatorial torus in the Earth's outer core: Evidence from the late-coda correlation wavefield. Science Advances, 10(35). https://doi.org/10.1126/sciadv.adn5562
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