Solar storm activity triggered an unusually strong geomagnetic disturbance on 19 January, lighting up the aurora borealis across the Nordic countries and parts of northern Europe, with sightings reported well beyond typical Arctic latitudes. Space-weather agencies said the event included a rare S4 solar radiation storm and a G4 geomagnetic storm, the strongest levels seen since the “Halloween” storms of 2003.
What happened on 19 January and why it was unusual
The disturbance began when a fast coronal mass ejection (CME) reached Earth and compressed the planet’s magnetic field, allowing charged particles to pour into the upper atmosphere near the poles. NOAA’s Space Weather Prediction Center said G4 (Severe) geomagnetic storm levels were first reached at 19:38 UTC on 19 January, and warned that elevated conditions could continue as the CME passed over Earth.
For observers, the practical effect was a larger-than-usual auroral oval: the northern lights became visible farther south than normal, including across large parts of northern and central Europe.
Why NOAA classified it as an S4 solar radiation storm
Alongside the geomagnetic storm, NOAA reported an S4 (Severe) solar radiation storm in progress on 19 January, based on measurements from the GOES-19 satellite. Solar radiation storms are driven by bursts of high-energy particles, mainly protons, accelerated during major solar eruptions.
This matters less for people on the ground—Earth’s atmosphere provides strong shielding—but it can increase operational risk for satellites, astronauts and high-latitude aviation, where radiation exposure and communication disruptions can become more significant.
Where the aurora was seen, from the Nordics to central Europe
In Finland, the Finnish Meteorological Institute said auroras were observed as far as the Alps, a sign of how intense the geomagnetic conditions were. In the Nordic region, cloud cover became the main limiting factor: forecasts suggested widespread clouds over much of Finland, with better chances in northern Lapland and in clearer pockets across northern Scandinavia.
For Denmark and southern Scandinavia, the event also stood out because auroras are typically faint at those latitudes and often only show clearly through a camera. During stronger storms, however, auroral structures can become visible to the naked eye even from light-polluted areas.
What a CME is, and what the storm scales mean
A coronal mass ejection (CME) is a large cloud of magnetised plasma ejected from the Sun. When a CME’s magnetic field couples efficiently with Earth’s, it can drive a geomagnetic storm.
NOAA uses separate scales for different types of space weather:
- G-scale (G1–G5) for geomagnetic storms that can affect power grids, navigation and radio communications.
- S-scale (S1–S5) for solar radiation storms that mainly affect satellites, astronauts and polar flights.
In this case, NOAA described both a G4 geomagnetic storm and an S4 solar radiation storm, which is why the event produced both a striking visual display and increased monitoring by operators of critical infrastructure.
Risks that rarely make the headlines: grids, GPS and polar routes
Most people experience space weather through photos of the aurora, but the same disturbances can create practical problems. During strong geomagnetic storms, induced currents can stress parts of the electricity network, and high-frequency radio links can degrade at high latitudes. Navigation systems can also suffer from increased ionospheric variability, affecting GNSS/GPS accuracy.
These effects are not constant everywhere and depend on local infrastructure and operational safeguards, but the combined severity levels reported by NOAA are one reason space-weather services issue alerts to industry and relevant authorities.
