S1 – Minor solar radiation storm in progress following M8.1 solar flare
An S1 – Minor solar radiation storm began at 17:00 UTC on May 31, 2025. The storm is linked to a long-duration M8.1 solar flare from Active Region 4100 at 00:05 UTC on May 31, associated with a strong Earth-directed coronal mass ejection (CME). Minor HF radio signal fades are being reported at high latitudes.
S1 - Minor solar radiation storm on May 31, 2025. Credit: SWPC
A minor solar radiation storm (S1) is currently in progress, following a long-duration M8.1 solar flare from Active Region 4100, resulting in a full halo CME.
Proton flux counts started rising around 08:00 UTC today and reached the S1 threshold at 17:00 UTC.
S1 events are the lowest on the NOAA solar radiation storm scale and typically cause only minor effects. During such events, shortwave radio users in polar regions may notice brief fades at lower frequencies, but no other significant impacts are expected for satellites, aviation, or ground-based infrastructure.
For most users, S1 events will go unnoticed, though polar radio operators and HF communication users may experience short-term signal degradation.
Active Region 4100 has remained a focus for forecasters, producing several M-class solar flares in recent days. The M8.1 flare observed earlier on May 31 was the most energetic since M8.9 on May 25. SWPC expects the active conditions to persist, with a continued likelihood of additional M-class flares and a slight chance for X-class events through June 2.
S1 – Minor solar radiation storm conditions could persist or recur over the coming days if strong solar activity continues. Elevated solar wind speeds from a coronal hole high speed stream are also influencing the near-Earth environment, maintaining an active geomagnetic field level.
CME produced by the M8.1 flare is expected to impact Earth on June 1, bringing the potential for strong to severe geomagnetic storming across high and mid-latitudes.
SWPC forecasters now predict that geomagnetic storm conditions are likely to reach G4 – Severe levels on both June 1 and June 2, with a decrease to G2 – Moderate levels expected on June 3, as the CME interacts with Earth’s magnetosphere.
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