‘Wow, this is something really new,’ said Xinlin Li, describing what the instruments observed in Earth’s near-space environment after an earlier, stronger solar storm changed the planet’s radiation belts in unexpected ways.’ This feeling of surprise is what helps to explain the appeal of January’s space weather event: it wasn’t just the aurorae, but how one solar event can be like two events at once. An X1.9 flare occurred in the vicinity of the sun’s equator, sending a fast coronal mass ejection (CME) towards Earth, triggering a geomagnetic storm of G4 intensity on the NOAA scale, and extending the auroral light well beyond its normal polar region.
On the surface, the situation appeared simple charged particles extended into the upper atmosphere and caused a widespread display of northern lights. But beneath the surface, it was a more complex engineering challenge. The transfer of energy from the storm to Earth’s magnetic field is very sensitive to the orientation of the magnetic field, especially the Bz component, which efficiently couples to Earth’s magnetic field when it is pointing south and provides a pathway for particles to flow in, while a switch to a north-pointing component causes the magnetosphere to close down. In this case, a brief period of southward pointing helped to nudge the storm into serious territory, followed by a longer period that restricted how much energy could continue to flow into the system, determining where aurorae would and wouldn’t appear. This disconnect between storm intensity and what is actually observed in the sky has become a familiar pattern of modern space weather.
The other side of the story was not as apparent but no less important. In addition to the geomagnetic storm, the outburst caused a solar radiation storm that reached S4 (“severe”) levels, a level that had not been reached since 2003. Radiation storms follow the path of energetic particles that are accelerated near the sun and released into interplanetary space; they can come quickly, accumulate at high latitudes, and increase radiation exposure for astronauts and certain air routes. It is a distinct stressor from geomagnetic disturbance, even if it comes from the same solar event.
For spacecraft mission operators, the concern is cumulative: radiation storms and geomagnetic storms can occur simultaneously with higher risk to satellites, communications, and navigation systems. Strong geomagnetic storms can cause charging and anomalies, and radiation storms introduce a particle environment that can damage electronics and sensors. Even if direct and overt disruptions are not significant, storms can identify design margins and where today’s margins are thin.
The most compelling reminder of this came from the benchmark event still lurking in the background: the “Mother’s Day storm” of May 2024, the most recent G5 geomagnetic storm, which changed the Earth’s radiation environment in ways that persisted for months. A small satellite, NASA’s CIRBE CubeSat, discovered two new provisional belts of high-energy particles in addition to the existing Van Allen belts. This is important because satellites bound for geostationary orbit make repeated passages through these regions, where high-energy particles can cause problems.
January’s storm did not have to approach the scale of May 2024 to be instructive. One eruption, with both a G4 geomagnetic storm and an S4 radiation storm, makes clear that intensity scale values are only part of the risk, while magnetic geometry and particle distributions determine the distribution of that risk across latitudes, across orbits, and across the technology that increasingly relies on stable near-Earth space.
