For nearly a decade ESA’s magnetic field mission has taken us on a journey from Earth’s core to the magnetosphere, and almost everything in between.

Consisting of identical triplets Alpha, Bravo and Charlie, Swarm was ESA’s first satellite constellation launched to observe Earth. It’s also the fourth Earth Explorer, one among a fleet of innovative pathfinders that each set new heights for Earth Observation.

The Swarm trio was tasked with measuring the invisible shield that protects Earth from cosmic radiation and space weather: the magnetic field generated by swirling, superheated iron within our planet’s core.

Each satellite carries a vector field magnetometer (VFM), which is calibrated by an absolute scalar magnetometer (ASM) - the most accurate magnetometer ever. The electric field instrument (EFI) is the first 3D ionospheric imager in orbit.

Alpha and Charlie orbit at a lower altitude of around 440 km, while Bravo cruises higher up at 510 km. Their position relative to each other allows for highly precise measurements with unprecedented resolution.

In 2018, the trio became a quartet with the addition of the e-POP instrument from Canada’s Cassiope satellite, which is now known as Swarm Echo.

Hugely successful, full of unexpected findings and in excellent shape with plenty of fuel in the tank, the mission has been extended to 2025, with big ambitions for the years ahead.

Getting to the core of the matter

The ever-lengthening time series of Swarm data, almost ten years and counting, is underpinning important advances in our understanding of Earth’s changing core.

Seven years’ worth of data have been used to map, in detail, the significant weakening of Earth’s magnetic field between South America and West Africa, known as the South Atlantic Anomaly. 

Magnetic north has been another focus of Swarm research, and smartphone users can thank Swarm for pinpointing it. It has been accelerating towards Siberia in recent years, a phenomenon that’s been put down to a tug of war between tussling magnetic blobs.

Other major findings include a jet stream in Earth’s core moving three times quicker than the rest of the outer core, and an explanation for weird jerks in Earth’s magnetic field that had previously puzzled scientists, but are now attributed to powerful hydromagnetic waves generated within Earth’s core.

Recently, an entirely new type of magnetic wave was discovered sweeping westward across the surface of the outer core at 1500 km a year, oscillating every seven years.

Duelling with the sun

Another major Swarm objective has been to map the response of the near-Earth environment to the solar cycle, which is ramping up at the moment to a predicted maximum in 2025.

That very same solar cycle puts the Swarm satellites themselves at risk. In the summer of 2022, the lower orbiting pair Alpha and Charlie had to be manoeuvred up through Earth’s atmosphere to combat drag caused by increasing solar wind

In late April 2023, an additional seven week orbit-raise campaign for the Alpha and Charlie commenced, as the sun continues to send powerful coronal mass ejections towards Earth. The first burns have been successful, and the new climb has begun.

More such climbs may be necessary in the coming months and years.

They are safe and well for now, and busy enabling new discoveries.

For example, whilst strong electric currents in the upper atmosphere were already known to vary by season, Swarm detected asymmetry between the north and south polar regions. More of the energy generated when charged particles from the solar wind flow into Earth’s atmosphere heads to the north pole than the south pole.

The extension to 2025 is particularly useful, as it means Swarm will have observed the response of near-Earth to an entire solar cycle.

Light chasing

One of the most visible effects of space weather and its interaction with Earth’s atmosphere is the aurora.

Whilst the Swarm trio feels the rearrangements in Earth’s magnetic field associated with its dazzling light displays, down below citizen scientists helped identify a new type of auroral feature known as Steve.

Linking up their pictures with Swarm data, Steve was attributed to a fast-moving stream of extremely hot atomic particles. Research on the effect continues.

Lightning chasers, too, have enlisted Swarm’s help to unravel the so-called transient luminous events that shoot lightning up into the ionosphere. Sprites, for example, discharge electricity at an altitude of around 50-90 km, above large thunderstorms.

Swarm data were able to link such events with fast magnetic field changes in the ionosphere.

Everything in between

Swarm mission has also helped get to the bottom of processes from the Earth’s surface down into its mantle.

It has long been suspected that migrating animals tap into Earth’s magnetic field lines to guide them. Scientists are now using Swarm mission data to map the flight paths of birds to Earth’s magnetic field, which may offer clues on migration behaviour.

The oceans over which migrating birds fly also proved a novel source of information. Swarm has been able to detect the elusive magnetic field generated by ocean tides, meaning scientists were able to image the electrical nature of Earth’s upper mantle. 

Such results help us to better understand plate tectonics. It is an area where Swarm data have had quite some impact, such as piecing together the tectonic history of Antarctica by providing magnetic measurements over hard-to-monitor, remote regions.

A major focus of the extension period to 2025 will be to further develop Swarm’s capabilities in understanding Earth’s lithosphere.

An ambitious future

“The Swarm mission has made great advances, and has a remarkable ability to surprise us with new uses,” says Swarm Mission Manager, Anja Strømme. “It enjoys fantastic community support in a wide and growing range of disciplines, and we have ambitious plans for the future.”

Those ambitions include an exciting project to introduce near-real time data. Known as FAST data, this will open up yet more opportunities.

For example, the thermospheric density data derived from Swarm provide extremely useful information on satellite drag. The assimilation of near-real time data is expected to improve the models that are necessary to avoid collisions between satellites.

Continuing its pioneering heritage, Swarm is also spearheading yet another ‘first’ for ESA. In January 2023, Swarm Bravo was the first satellite to be commanded by ESA’s new Ground Operation System Common Core (EGOS-CC).

In late 2023, Swarm will become the first mission to entirely migrate to this new system, heralding a new era. From 2025, all new missions will be entirely controlled by EGOS-CC, which replaces the current SCOS-CC system.

“It is fantastic to see this multifaceted mission continue to make its mark,” says Anja. “We look forward to many more ‘firsts’ in the years ahead.”