Scientists have long studied Earth’s core using seismic waves generated by earthquakes, which act like an ultrasound. In a recent study, geophysicist Hrvoje Tkal?i? and his colleague Xiaolong Ma uncovered a surprising new feature, a donut-shaped region in the core near the Equator where seismic waves slow down.
Traditionally, seismic studies have focused on the large, initial waves generated by earthquakes. However, Tkal?i? and Ma took a different approach. By analyzing the fainter, later part of these waves, known as the coda, they made a groundbreaking discovery.
“We detected tiny signals stemming from multiple reverberating waves we wouldn’t otherwise see,” they explained, referring to their use of a new technique called the “coda-correlation wavefield.”
Through this method, the researchers found that seismic waves near the Equator travel around 2% slower than those closer to the poles. This discrepancy pointed to a large, donut-shaped region in the outer core, just a few hundred kilometers thick, where seismic waves behave differently.
“There must be a torus – a donut-shaped region – in the outer core around the Equator,” said the researchers.
The region’s slower wave speed suggests it contains a higher concentration of lighter elements, such as silicon and oxygen. These elements play a crucial role in the flow of liquid metal within the core, which in turn generates Earth’s magnetic field.
“We think this region contains lighter elements and may play a crucial role in the vast currents of liquid metal running through the core,” said Tkal?i? and Ma.
Earth’s outer core is primarily composed of iron and nickel, but it also contains lighter elements like silicon, oxygen, and sulfur. These lighter elements are thought to migrate from the solid inner core into the outer core, where they influence the flow of liquid metal. This movement, driven by thermal convection, helps sustain the planet’s magnetic field, which shields the Earth from harmful solar radiation.
One of the more intriguing aspects of the discovery is that previous research suggested waves moved more slowly throughout the entire “ceiling” of the outer core. However, Tkal?i? and Ma’s study shows that the low-velocity region is concentrated near the Equator. The researchers believe this is linked to the buildup of lighter elements in this specific area, likely driven by heat transfer between the outer core and the rocky mantle above it.
In addition to the flow of liquid metal, the Earth’s rotation and its small, solid inner core organize the outer core’s movement into long, vertical vortices, similar to giant waterspouts. These vortices contribute to the “geodynamo” effect, responsible for generating and maintaining the Earth’s magnetic field.
“This magnetic field shields the planet from harmful solar wind and radiation, making life possible on the surface,” the study stated.
