Swarm detects ocean tides’ magnetic signatures

A study using data from ESA’s Swarm mission suggests that faint magnetic signatures created by Earth’s tides can help us determine magma distribution under the seabed and could even give us insights into long-term trends in global ocean temperatures and salinity.

Swarm is a constellation of three satellites that study Earth’s geomagnetic field. This magnetic field that extends from Earth’s interior into space is thought to be produced largely by an ocean of liquid iron in the planet’s outer core. Other sources of magnetism include magnetized rocks in the crust.

And although we might not normally think of oceans as generating magnetism, the salty sea water is a moderate electrical conductor. This means that as tides flow across Earth’s magnetic field, they generate weak electric currents, which in turn induce small magnetic signals that can be detected from space.

With its satellites flying at an altitude between 462 km and 511 km, Swarm measures Earth’s magnetic field more accurately than ever before. It can detect faint tidal signatures and distinguish them from other stronger magnetic field sources from Earth’s interior.

“This study shows that Swarm can provide data on properties of the entire water column of our oceans.” says Anja Strømme, ESA’s Swarm Mission Manager.

Swarm’s data can also provide insights into the distribution of magma, which could in future support better understanding of events such as the Hunga-Tonga volcanic eruption of 2022.

The study of these signatures made the front cover of the world’s oldest scientific journal, Philosophical Transactions of the Royal Society A, and was conducted by a team from the University of Cologne and the Technical University of Denmark.

Swarm gets better with age

The mission, launched in 2013, was only meant to fly for four years but is now in its 12th year. Anja adds, “This is one of the benefits of flying missions for longer than originally planned. So, by flying as long as the scientific output is of excellent quality and resources allow, you can tackle scientific questions that weren’t originally envisaged.”

Swarm is, however, slowly nearing the natural end of its lifespan as drag gradually brings the satellites physically closer to Earth. This has enabled the mission’s instruments—the satellites carry state-of-the-art sensors including magnetometers that measure the strength, magnitude and direction of the magnetic field—to capture faint signals that would be more difficult to detect from the higher orbits at the start of the mission.

Less solar interference

Swarm’s ability to detect the faint ocean signals was also helped by the sun’s less active period around 2017. “These are among the smallest signals detected by the Swarm mission so far,” says lead author Alexander Grayver, of the University of Cologne.

“The data are particularly good because they were gathered during a period of solar minimum, when there was less noise due to space weather.”

The “minimum” period of the sun’s 11-year solar cycle is when the sun’s surface is least active. During this “quiet” period, it emits less solar matter—including electromagnetic radiation and charged particles—so “space weather” phenomena such as the Northern Lights are less frequent. And with less electromagnetic radiation from the sun, the geomagnetic signals from Earth are more easily detectable by Swarm’s magnetometers and other instruments.

The hope is that, when the next solar minimum comes around after 2030, Swarm might still be flying—albeit at a lower altitude—and will be able to continue detecting the faint signals that can help us understand more about the temperatures and salinity deep within our oceans.