Very rare S4 – Severe solar radiation storm in progress, G4 – Severe geomagnetic storm forecast

Very rare S4 – Severe solar radiation storm in progress, G4 – Severe geomagnetic storm forecast

Teo Blašković

Severe solar radiation storm is in progress after ≥10 MeV proton flux exceeded 10 000 pfu at 18:10 UTC on January 19, 2026, following the X1.9 solar flare on January 18. As the associated full-halo coronal mass ejection approaches Earth, forecasters warn that a G4 – Severe geomagnetic storm is likely early on January 20.

The solar radiation storm intensified rapidly through January 19, reaching S4 – Severe levels at 18:10 UTC as high-energy proton flux crossed the 10 000 pfu threshold.

The event is associated with a long-duration X1.9/3b solar flare that peaked at 18:09 UTC on January 18. The eruption produced Type II and Type IV radio emissions, a 3 200 sfu F10.7 radio burst with a Castelli-U signature, and a full-halo coronal mass ejection observed shortly after the flare peak, indicating an Earth-directed event capable of sustained particle acceleration.

olar radiation storms reaching S4 – Severe intensity are very rare. Such events typically occur only a small number of times per 11-year solar cycle, while in weaker cycles they may be entirely absent. Most proton events peak at S1 or S2 levels, with only a limited subset progressing through S3 and into S4, generally in association with the most energetic solar eruptions and fast, Earth-directed CMEs.