The European Space Agency's three-satellite Swarm constellation is three years into its four-year study of the Earth's magnetic field. Back in May, a study used Swarm data to show where and how that field has changed over the last few years, and now the project has discovered the driving force behind them: wrapped around the outer core of our planet is a geological "jet stream" made of molten iron that flows at tremendous speeds – and it's getting faster.

The Earth's magnetic field plays a crucial role in shielding our planet from harmful cosmic rays and solar wind. Life simply wouldn't exist without it, as it would strip the planet's precious atmosphere and leave Earth an inhospitable wasteland, like our rocky red neighbor. This life-saving field has long been thought to be the result of liquid iron flowing around the core, some 3,000 km (1,864 miles) below our feet.

With so much rock blocking our view, studying the magnetic field is one of the only ways we can determine what's happening down there. Swarm's unique ability to separate the different types of magnetic signals from various parts of the planet, from the inner core to the upper magnetosphere, has helped shed more light on the Earth's hidden workings, including finding this jet stream moving within that liquid layer.

"The European Space Agency's Swarm satellites are providing our sharpest x-ray image yet of the core," says Phil Livermore, lead researcher on the study. "We've not only seen this jet stream clearly for the first time, but we understand why it's there. We can explain it as an accelerating band of molten iron circling the North Pole, like the jet stream in the atmosphere."

This underground jet stream is focused in the northern hemisphere, particularly underneath Alaska and Siberia, where it reaches a top speed of over 40 km (25 mi) per year. Compared to other geological movements, that's an absolute sprint: it's three times faster than most other outer core elements, and hundreds of thousands of times faster than the tectonic plates we're riding at the Earth's crust.

The researchers found that the jet stream most likely straddles the boundary between two different parts of the core. Liquid iron is forced out from both of those sections towards this boundary, where it's then squeezed outwards and pushed around at speed.

"Of course, you need a force to move the liquid towards the boundary," says Rainer Hollerbach, co-author of the study. "This could be provided by buoyancy, or perhaps more likely from changes in the magnetic field within the core."

The researchers plan to continue to watch how the magnetic field changes over time, and are expecting some more surprises , including the possibility that the jet stream could even switch direction.

"This feature is one of the first deep-Earth discoveries made possible by Swarm," says Rune Floberghagen, the Swarm mission manager at ESA. "With the unprecedented resolution now possible, it's a very exciting time – we simply don't know what we'll discover next about our planet."

The research was published in the journal, Nature Geoscience.

An animation of the Swarm satellites can be seen below. (Video: ESA/AOES Medialab)

Sources: University of Leeds, European Space Agency

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