Each star tracker consists of three cameras pointing in different directions. These cameras are sensitive to ionising radiation: when an energetic particle hits the sensor, it appears as a white spot. Analysts have been able to produce data on particle fluxes, in particular detecting impacts from particles with energy levels of Megaelectronvolt (MeV), higher than 100 MeV. While the South Atlantic Anomaly (SAA) is the main source of penetrating ionising radiation, magnetic storms can lead to a temporary enhancement of high-energy particles (protons and electrons) in polar regions. These particles are injected into the magnetosphere and becoming trapped. This image shows the aurora borealis over the North Pole on 12 November, 2025. Credit: CSU/CIRA & NOAA

The European Space Agency’s Swarm mission detected a large but temporary spike of high-energy protons at Earth’s poles during a geomagnetic storm in November. It did this not with the scientific instruments for measuring Earth’s magnetic field, but with its ‘star tracker’ positioning instruments—a first for the Swarm mission.

While Swarm’s magnetometers detected magnetic fluctuations 10 times stronger than normal on 12 November, it was the star trackers that detected a temporary increase in high-energy protons around the poles. During the geomagnetic storm of 11–13 November, levels of high-energy proton flux were 300 times higher than normal levels.

Measuring the magnetosphere

Swarm, an Earth Explorer mission developed under ESA’s Earth Observation FutureEO program, is dedicated to understanding more about the invisible force field around our planet. Earth’s magnetic field reaches from deep inside the planet’s molten core and extends far out into space, protecting us from cosmic radiation and solar winds by deflecting harmful charged particles.

Orbiting at an altitude of 400–500 km, the Swarm satellites are perfectly positioned to monitor the effects of geomagnetic storms.

Each of the three Swarm satellites, launched together in 2013, carries several instruments, including two types of magnetometer, which are able to measure both the strength and the direction of the magnetic field. They also carry star trackers to ensure correct location and orientation in space.

This GIF shows the maximum flux value observed by each camera at a given timestamp. While the South Atlantic Anomaly (SAA) is the main source of penetrating ionising radiation, magnetic storms can lead to a temporary enhancement of high-energy particles (protons and electrons) in polar regions. These particles are injected into the magnetosphere and becoming trapped. Credit: ESA

Star trackers are optical instruments that measure a satellite’s position and attitude (orientation) by determining its position in relation to stars. So, while star trackers are normally used to correctly position satellites in space, in this instance, Swarm’s star trackers became a surprising source of important data.

The November solar event

Between 11–13 November 2025, Earth was hit by an exceptionally strong solar storm, caused by three consecutive coronal mass ejections within 48 hours.

These gave rise to ’"roton auroras," which appear as a more diffuse light or glow in the sky and are typically seen at much lower latitudes during strong storms. Electron auroras, on the other hand, are associated with the appearance of ‘ripples’ of light in the sky and are often at higher latitudes.

While geomagnetic storms cause beautiful aurora, the charged particles emitted by the sun’s flares can pose a threat to infrastructure on Earth, with the potential to interrupt and damage energy grids and communications. On this occasion, a short radio blackout was recorded across Europe, Africa and Asia, lasting approximately 30–60 minutes.

High-energy proton flux over the polar regions. Credit: ESA

What Swarm saw: From stars to protons

On 12 November, the star trackers detected a huge influx of high-energy protons entering the polar regions. During severe geomagnetic storms, Earth’s magnetic shield becomes disturbed, allowing a much greater number of energetic particles to reach low-Earth orbit—and in this case, the flux was unusually intense. This high-energy solar proton event is a rare phenomenon.

While they’re not a danger to people on Earth, high-energy protons can severely disrupt and damage spacecraft electronics, including solar cells, and are hazardous to human spaceflight.

"This is a fascinating use of Swarm’s star trackers, which are normally used to correctly orient the satellites," said ESA’s Swarm Mission Manager, Anja Stromme. "The high-energy particle product is a newly implemented functionality for Swarm, and the products will be released operationally on 17 December. This is therefore the first event where a space weather event is monitored by Swarm’s star trackers."

The star tracker image sensors are sensitive to high-energy protons. When one hits the sensor, it appears as a white spot on the image. While this is normally considered an inconvenience, the spots can also record the flux of energetic protons with energy higher than 100 MeV.

High-energy protons, in the form of ionizing radiation, normally penetrate Earth’s magnetic field at the South Atlantic Anomaly—an area covering part of the Atlantic Ocean and South America, where Earth’s magnetic field is weaker. During magnetic storms, however, protons can travel into Earth’s magnetosphere and become trapped. This process can lead to a temporary enhancement of high-energy particles in the polar regions, as observed in this case.

According to Enkelejda Qamili, a Swarm data quality analyst at ESA, the elevated proton levels demonstrate how low-Earth orbit missions can effectively monitor and detect solar particle events, highlighting the continued high activity of the sun.

"Under normal conditions, Earth’s magnetic field deflects most solar wind particles; however, during a geomagnetic storm, the magnetosphere can become overloaded, allowing a substantial number of high energy protons to penetrate and give rise to several geophysical phenomena. While these events are of great scientific interest, it is important to acknowledge the potential risks they pose to astronauts, spacecraft and communication."

Citation: Swarm detects rare proton spike during solar storm (2025, December 11) retrieved 11 December 2025 from https://phys.org/news/2025-12-swarm-rare-proton-spike-solar.html

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