Earth was just hit by the strongest solar radiation storm in over 20 years — here's what it means

While a severe G4 geomagnetic storm impressed skywatchers with vivid auroras around the world this week, a far less visible, but historically significant, space weather event was also underway.

On Monday (Jan. 19), Earth experienced its strongest solar radiation storm since October 2003, according to NOAA's Space Weather Prediction Center, surpassing the intensity of the notorious October 2003 "Halloween" space weather storms.

Solar radiation storms occur when a powerful magnetic eruption on the sun, often involving a coronal mass ejection (CME), accelerates charged particles, mainly protons, to extreme speeds. These particles can reach a significant fraction of the speed of light, allowing them to traverse the roughly 93 million miles (150 million kilometers) between the sun and Earth in tens of minutes or less, according to NOAA. When they arrive, the most energetic protons can penetrate Earth's magnetic defenses and travel along our planet's magnetic field lines toward the polar regions, where they plunge into the upper atmosphere.

NOAA classifies solar radiation storms on a scale from S1 (minor) to S5 (extreme) based on GOES satellite measurements of incoming high-energy protons. The Jan. 19 event reached S4 (severe) levels.

While it may sound dramatic, this type of storm poses no threat to people on the ground, thanks to Earth's thick atmosphere and magnetic field, which absorb the radiation before it reaches the surface.

Notably, this was not a "ground-level event," in which particles are energetic enough to be detected at Earth's surface. As space weather physicist Tamitha Skov explained, this storm had a relatively "soft" particle spectrum — historic in strength, but lacking the extreme energies needed to reach the ground.

High above the surface, it's a slightly different story.

Severe radiation storms increase exposure risks for astronauts and for airline crews and passengers flying along polar routes, where Earth's magnetic shielding is weaker. Satellites are also vulnerable: energetic particles can interfere with onboard electronics, disrupt sensors, and overwhelm instruments. During this storm, some space weather forecasters reported temporary data dropouts, likely caused by intense proton fluxes degrading spacecraft measurements.