The researchers chose to focus on the oceans because they make a tiny, but important contribution to the Earth's overall magnetic field. The salt within seawater can conduct electricity. And oceans don't remain still; rather, they move in cycles, up and down. As the tides cycle through the world's oceans, that salty water essentially tugs on the magnetic field above our planet.
"We have used Swarm to measure the magnetic signals of tides from the ocean surface to the seabed, which gives us a truly global picture of how the ocean flows at all depths — and this is new," Nils Olsen, the head of geomagnetism at the Technical University of Denmark, said in a statement.
The magnetic field generated by the oceans is quite small. "It's about 2 [to] 2.5 nanotesla at satellite altitude, which is about 20,000 times weaker than the Earth's global magnetic field," Olsen told BBC News.
The newly analyzed data will give researchers a more nuanced view of how the oceans are affected by climate change, Olsen noted. "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," Olsen said in the statement.
Moreover, the tidal magnetic signal, in turn, induces a weak magnetic response from deep under the seabed, he said.
The research is the latest discovery Swarm has gathered with respect to Earth's magnetic field. Last year, researchers announced that the spacecraft had helped map magnetic signals emitted by Earth's outer shell, known as the lithosphere, Live Science previously reported.
The new research, which has yet to be published in a peer-reviewed journal, was presented yesterday (April 10) at the 2018 European Geosciences Union meeting in Vienna.