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Underneath our feet, thousands of kilometers deep, molten iron and nickel move about in the Earth’s outer core. This motion creates our planet’s magnetic field – a shield that has protected life against the worst of cosmic radiation for billions of years. Understanding the geomagnetic field is still a work in progress, and new research from the European Space Agency reveals magnetic waves that exist on the surface of the outer core, at the boundary with the mantle.
As reported in the Proceedings of the National Academy of Sciences, the wave sweeps slowly across the outer core at 1,500 kilometers per year (0.1 miles per hour). And it goes around every seven years.
“Geophysicists have long theorised over the existence of such waves, but they were thought to take place over much longer time scales than our research has shown,” lead author Dr Nicolas Gillet, from the University Université Grenoble Alpes, said in a statement.
“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.”
The space data comes from the Swarm mission as well as some older data from the German Champ and Danish Ørsted missions. Swarm is made up of three identical satellites that can measure magnetic fields in the core, as well as signals from other regions of the planet, from all the way up in space.
The data suggests that the waves are strongest at the equator and they are aligned in columns along the planet’s axis of rotation. Just like the Coriolis force affects the motions of fluids on the planet north and south of the equator, these waves are exhibiting a form of magneto-Coriolis motion.
“Magnetic waves are likely to be triggered by disturbances deep within the Earth's fluid core, possibly related to buoyancy plumes. Each wave is specified by its period and typical length-scale, and the period depends on characteristics of the forces at play. For magneto-Coriolis waves, the period is indicative of the intensity of the magnetic field within the core,” Dr Gillet added.
“Our research suggests that other such waves are likely to exist, probably with longer periods – but their discovery relies on more research.”
The research provides new insight into the behavior of the core and maybe more. Given the location of the waves – just at the bottom of the mantle – studying them might lead to a new understanding of the lower portion of the mantle, including its electrical conductivity.
