Swarm’s surprising source of energetic proton data

Here we see some interesting data from ESA’s magnetic field mission, Swarm, making use of a surprising source of information on energetic protons.

Swarm’s star trackers, which are normally used to provide highly accurate spacecraft attitude (orientation in space), detected a temporary enhancement of high energy protons at the poles during the back-to-back space weather events of 12 and 13 November 2025.

The cameras on the star trackers are sensitive to energetic particles, and scientists have developed a Swarm product that provides the flux of protons with energy higher than 100 MeV. These are most frequently observed by Swarm over the weak area of Earth’s magnetic field known as the South Atlantic Anomaly, but protons can also be injected into the magnetosphere and trapped during magnetic storms.

The data you see in the gif were taken from Swarm Bravo, the higher orbiting satellite of the 3-satellite constellation. Each star tracker consists of three camera head units (CHUs), orientated orthogonally to maximise the overall attitude accuracy and prevent simultaneous blinding. The image sensors inside each CHU are sensitive to ionising radiation: when an energetic particle impacts the sensor, it appears on the source image as white spot, known as an ‘energetic particle detection’. Here, we show the maximum flux value observed by each CHU at a given timestamp.

High energy protons give rise to “proton auroras”, a more diffuse aurora lacking the dancing displays often associated with electron auroras, that can be seen at much lower latitudes during strong geomagnetic storms. Although not harmful for people on the ground, solar proton events can interfere with or even damage spacecraft electronics and solar cells. Understanding them can help to reduce risks to satellites and human spaceflight.