As it transits through the skies above, the Moon’s pull on the ocean’s salty depths generates a second, if much weaker, global magnetic field.

The ebb and flow of salty water, caused by our Moon’s gravitational pull, can induce their own magnetic field — one which a trio of European Space Agency’s (ESA) satellites has mapped in exquisite detail.

Known as “Swarm”, the trio of satellites was blasted off into orbit back in 2013 to help us better understand the planet’s magnetic field. Most of that field is produced by the churnings of molten iron in the Earth’s core, functioning like a massive underground dynamo. There are other secondary effects, however, such as those produced by human activity — and those are the effects Swarm was intended to peer into.

Imagine the surprise among ESA’s researchers when the satellites stumbled into a whole new magnetic phenomenon.

“It’s a really tiny magnetic field. It’s about 2-2.5 nanotesla at satellite altitude, which is about 20,000 times weaker than the Earth’s global magnetic field,” Nils Olsen, from the Technical University of Denmark, told BBC News.

What set the satellite trio apart from its peers — and enabled this discovery — is the way they ‘see’ water. Other devices we’ve sent in orbit record tides as a change in sea-surface height, but Swarm’s magnetic instruments view the movements of the entire column of water, all the way down to the seabed.

Water is diamagnetic, meaning that it has weak magnetic qualities when a magnetic field is applied to it. However, adding salt reduces its diamagnetism but makes it a good, but not great, electrical conductor — meaning it will start interacting with magnetic fields, relatively weakly. Still, oceans house humongous quantities of water, and as tides cycle around ocean basins, the overall effect is enough to ‘pull’ the geomagnetic field lines along. The interaction between saltwater and the Earth’s magnetic field also generates electrical currents, which, in turn, induce their own magnetic signals.

Studying the ebb and flow of this second magnetic field can let us peer into the movement of deep bodies of water. Oceans capture, store, and move a lot of heat around, and Swarm’s findings could help researchers build better models of Earth’s systems — particularly useful in understanding the effects of climate change.

The magnetic signature of the tides causes a “weak magnetic response” deep below the sea, Olson explained — which could allow us to peer into the electrical goings-on of our planet’s lithosphere and upper mantle. Such data will help us better map these structures, as well as the tectonic activity that drives earthquakes and volcanic eruptions.

“Since oceans absorb heat from the air, tracking how this heat is being distributed and stored, particularly at depth, is important for understanding our changing climate,” Olson said in a statement, adding that the discovery “gives us a truly global picture of how the ocean flows at all depths.”

The professor was speaking at the European Geosciences Union General Assembly (EGU) in Vienna, Austria, where a clutch of new Swarm results have been released.

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