The bowels of the Earth are far from the quietest place. Deep below our surface, the planet is buzzing with activity, from plate tectonics to convection currents circulating in hot magmatic fluids far below the crust. When studying satellite data about the Earth, scientists have discovered inside the Earth what we have never seen before: a new type of magnetic wave that envelops the surface of the core of our planet every seven years. This discovery may give an idea of how the Earth’s magnetic field is generated, and of the thermal history and evolution of our planet, that is, of the gradual cooling of the planet’s interior.
“Geophysicists have long theorized over the existence of such waves, but they were thought to take place over much longer time scales than our research has shown,” says geophysicist Nicolas Gillet of the Université Grenoble Alpes in France. “Measurements of the magnetic field from instruments based on the surface of Earth suggested that there was some kind of wave action, but we needed the global coverage offered by measurements from space to reveal what is actually going on. We combined satellite measurements from Swarm, and also from the earlier German Champ mission and Danish Ørsted mission, with a computer model of the geodynamo to explain what the ground-based data had thrown up – and this led to our discovery.”
The Earth’s magnetic field is of great interest to scientists. Studies show that the invisible structure forms a protective “bubble” around our planet, not allowing harmful radiation inside, and the atmosphere outside, which allows life to prosper.
But the magnetic field is not static. It fluctuates in strength, size, and shape. It also has features that we do not understand, and gradually weakens over time. The reason why activity inside our planet is important is that this is where the magnetic field comes from. It is generated by a dynamo – a rotating convective and electrically conductive fluid that converts kinetic energy into magnetic energy by spinning a magnetic field out into space around the planet. This fluid is (mostly) molten iron inside the outer core of the Earth.
The European Space Agency’s (ESA) Swarm satellites are three identical probes launched in 2013 into orbit to study activity inside the Earth, with a focus on magnetic and dynamic activity emanating from the nucleus. It was in this data that Gillet and his team discovered exciting new waves.
They then studied data from other terrestrial and space observatories collected from 1999 to 2021 and found patterns. These waves, known as magneto-Coriolis waves, are huge magnetic poles aligned along the Earth’s axis of rotation, strongest at the equator. They cover the boundary between the core and the mantle with an amplitude of about 3 km per year and move west at speeds up to 1500 km per year. Their existence suggests that there may be other magneto-Coriolis waves with other periods of oscillation, which we can not detect today due to a lack of data.
At the moment, because waves carry information about the environment they travel through, the new discovery can be used to explore the interior of our planet in new ways. Including the nucleus, which is difficult to study, and the boundary between the core and mantle.
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