The European Space Agency (ESA) recorded one of the most striking particle “attacks” on Earth’s magnetic shield in recent years during a powerful solar storm in November 2025. A key role was played by the Swarm mission, a constellation of three satellites dedicated to precisely measuring Earth’s magnetic field. Alongside the usual scientific instruments, navigation cameras – so-called star trackers – unexpectedly jumped to the forefront this time, as instead of stars, they “saw” a surge of high-energy protons from a solar eruption.

The storm was the result of the exceptionally active solar region NOAA AR 14274, which between November 11 and 14, 2025, emitted four powerful solar flares and an equal number of coronal mass ejections (CMEs), three of which were directed toward Earth. The strongest flare, class X5.1, erupted on November 11, and the accompanying CME reached our planet the following day around 18:50 UTC, triggering a strong geomagnetic storm that shook Earth’s magnetic sheath for several hours.

Although this episode did not cause serious damage to ground infrastructure, it brought two key lessons: how quickly the Sun can change conditions in our space neighborhood and how precious data from missions like Swarm are for understanding and predicting space weather.

Swarm – a magnetic stethoscope for Earth

Swarm is an ESA mission launched in 2013 as part of the Earth Explorer program, designed to detailly measure the structure and changes of Earth’s magnetic field with three identical satellites in polar orbit. The satellites fly at altitudes of approximately 450 to 530 kilometers and continuously map the contribution of the core, mantle, lithosphere, oceans, and ionosphere to our planet’s total magnetic signal.

Each satellite carries several key instruments: vector and scalar magnetometers for measuring magnetic field strength and direction, electric and plasma probes, accelerometers, and laser retroreflectors for precise orbit determination. In the background of the operation of almost all these instruments are star trackers – optical cameras that photograph the sky and, by comparing the position of stars with a built-in catalog, constantly calculate the satellite’s orientation in space.

These navigation systems are usually “invisible” to the public because they have no direct scientific role. But they became the unexpected heroes of November 2025, turning into improvised radiation detectors that engineers knew how to utilize.

November 2025: a storm of three coronal ejections

The period around November 11, 2025, unfolded at a time when the Sun had already entered the solar maximum phase, the peak of the 11-year activity cycle in which powerful flares and coronal ejections are more frequent. Within less than 48 hours, the active region NOAA AR 14274 ejected three consecutive CMEs toward Earth. The combination of these ejections created conditions for a strong geomagnetic storm when the main plasma impact hit Earth’s magnetic field on November 12.

At moments of peak activity, the geomagnetic storm caused significant fluctuations in the magnetosphere and upper layers of the atmosphere. ESA models and satellite measurements showed that magnetic irregularities in the early phases of the storm intensified up to ten times compared to usual values, which is a very clear signal that Earth’s defense shield is under strong pressure from the solar wind and the CME shock front.

The most immediate consequence was a serious disruption of radio communications. In areas that were illuminated by the Sun at the time of the flare eruption – primarily in Europe, Africa, and Asia – a strong radio blackout on shortwave frequencies was recorded, lasting roughly between 30 minutes and one hour. Such events directly affect long-distance aviation routes, maritime communication, and part of military systems that depend on the ionosphere as a reflecting layer for radio waves.

Another indicator of the exceptional strength of this storm was a rare Ground Level Enhancement (GLE) – an increase in the flux of high-energy particles strong enough for some to penetrate deep into the atmosphere and be recorded by cosmic ray detectors on the ground. Statistics say that such events are recorded only once or twice a year and that this November GLE is only the 77th recorded since the 1940s, which further emphasizes its exceptional nature.

From stars to protons: how star trackers became radiation detectors

While Swarm’s magnetometers neatly recorded every “tremor” of the magnetic field, the star trackers registered something completely different during the storm: a bombardment by high-energy protons. Each tracker consists of three mutually perpendicularly positioned “camera head” modules, whose sensors are also sensitive to ionizing radiation. When a proton of sufficiently high energy passes through the sensor, it leaves a characteristic white dot on the image, a so-called energetic particle detection.

Such “dots” are normally just a nuisance for algorithms looking for clean star patterns for navigation. However, Swarm engineers had earlier developed software that converts the counting of these detections into data on the proton flux over a specific location. In an orbit of about 500 kilometers, where the Swarm Bravo satellite with the highest trajectory in the constellation is located, star trackers can thus continuously measure how often protons with energies greater than 100 MeV pass through their sensors.

During the storm in November 2025, it was precisely on the basis of this data that an exceptional intensification of the proton flux over polar regions was recorded. As Earth’s magnetic shield was temporarily “crumpled” and compromised, a portion of the high-energy particles that otherwise remain trapped in the mutual “tangle” of magnetic lines or are deflected by the magnetic field away from the planet managed to penetrate to low Earth orbit altitudes.

Swarm’s star trackers thus served for the first time in operational practice as a kind of real-time proton detection network. Data from this event represent one of the first demonstrations of the new Swarm product for monitoring high-energy particles, which has recently been used for more detailed monitoring of solar activity from a low orbit perspective.

Proton auroras and the “invisible” threat to space infrastructure

One of the most interesting visual effects observed during this storm were the so-called proton auroras. Unlike “classic” polar lights mostly caused by electrons, which draw dynamic curtains, arcs, and “swirls” of light in the sky at very high geographical latitudes, proton auroras appear as a diffuse, uniform light haze. In powerful storms, they can descend to much lower latitudes than usual, so during November 2025, they were also recorded in areas where residents rarely have the opportunity to see an aurora with their own eyes.

Physically speaking, it is the same process: charged particles from the solar wind, guided by Earth’s magnetic field lines, enter the upper atmosphere and collide with oxygen and nitrogen molecules, exciting them and prompting them to emit light. In proton auroras, protons play the dominant role, so energy is transferred in a somewhat different way and with a different distribution by altitude and geographical latitude.

For people on the ground, including passengers in aircraft at usual flight altitudes, such events do not pose an immediate health risk. Likewise, a strong GLE like this one from November 2025 is still far below levels that would require emergency measures for the population. But for satellites and astronauts, high-energy protons represent a serious problem: they can damage solar cells, accelerate the aging of electronic components, disrupt logic circuits, or cause temporary “bit flips” in memory.

That is why space agencies, including ESA’s Space Weather program, implement a series of measures at times of heightened solar activity – from adjusting the pointing directions of sensitive instruments to postponing critical maneuvers and protecting astronauts in better-shielded parts of spacecraft. The principle they strive for is ALARA (“as low as reasonably achievable”) – reducing radiation exposure to the lowest possible level, given real operational limitations.

South Atlantic Anomaly – a natural “window” for radiation

Although the November storm temporarily increased the proton flux at the poles, Swarm has for years been recording high-energy particles over another, chronically problematic area: the South Atlantic Anomaly (SAA). This is a vast region over the southern Atlantic and part of South America where Earth’s magnetic field is significantly weaker than elsewhere, so the inner Van Allen radiation belt descends much closer to the planet’s surface.

In that area, satellites in low orbit pass through a “pocket” of intensified radiation. The reason is the geometry of Earth’s magnetic dipole, which is not perfectly centered relative to the axis of rotation. The consequence is that magnetic field lines in the SAA area are more rarefied and less effective at repelling charged particles. Result: a greater flux of protons reaching the altitude of typical orbits of missions like Swarm or the International Space Station.

Swarm data showed that the SAA has been changing in recent years – growing, shifting, and changing its internal structure, which points to complex dynamics in Earth’s core and mantle. Scientists use this information to improve magnetic field models and better predict how areas of intensified radiation will develop in the future. For satellite operators, this means the possibility of more precise planning for passing through the anomaly, optimizing orbits, and protecting electronics.

Why this storm is important for future missions

The November event of 2025 is an ideal example of how a combination of different missions and instruments provides a complex but exceptionally valuable picture of a space weather event. SMOS, for example, registered a strong solar radio burst almost 14 hours before the CME impact, Swarm measured magnetic fluctuations and proton flux, while missions like SOHO, Solar Orbiter, and BepiColombo tracked the storm’s development in interplanetary space.

At the same time, the analysis of such events highlights the limitations of early warning systems. Current satellites located near the L1 Lagrange point give operators only about twenty minutes of warning between the detection of an incoming CME and its impact on Earth’s magnetic field. ESA is therefore developing a new generation of missions for space weather monitoring, such as the Vigil satellite, which will observe the Sun from the side at the L5 position and detect potentially dangerous ejections earlier.

For space exploration beyond the safe haven of Earth’s magnetic shield – toward the Moon, Mars, and beyond – understanding the behavior of high-energy protons and the effectiveness of planetary magnetic protection becomes a matter of crew safety and equipment durability. Every event like the November 2025 storm serves as a natural “stress test” of our models and technologies.

For the wider public, the most visible trace of the storm remains the spectacular auroras that this time descended unusually far south, offering residents of Europe, and even the Mediterranean, a rare opportunity to watch for several hours as solar activity drew light curtains high above the horizon. For scientists and engineers, more important is the silent record in the telemetry of Swarm and other missions – data that will help in the coming years to develop more precise space weather forecasts and more robust space systems.

The Swarm mission, conceived more than a decade ago as a “magnetic stethoscope” for Earth, thus showed in November 2025 that it can also be a sensitive dosimeter for high-energy protons. At a time when the solar maximum increases risks for satellites, communications, and future human missions into deep space, every such additional source of data becomes an invaluable tool for understanding and protecting our technological society.