High-energy solar protons briefly reached S1 – Minor solar radiation storm levels on May 26, 2026, following a large partial halo coronal mass ejection (CME) from the far side of the Sun. The eruption was first detected by the GOES-19 CCOR-1 coronagraph at 22:00 UTC on May 25. Forecast calls for a 10% chance of another S1 or stronger solar radiation storm on May 27.
Proton levels were already falling by the start of May 27 after reaching S1 solar radiation storm levels on May 26. The event was linked to a large far-sided partial halo CME. The decline slowed after about 17:15 UTC on May 26 when the greater than 10 MeV proton flux averaged about 1 pfu, below NOAA’s S-scale storm threshold.
By 12:30 UTC on May 27, solar radiation levels observed by GOES-19 over the previous 24 hours were below storm levels. Elevated background proton levels kept a small chance of renewed S1 or stronger conditions in the latest forecast, with probabilities of just 10% on May 27, 5% on May 28, and 5% on May 29.
Solar activity remained low during the same period over the past 24 hours. The strongest flare was a C9.7 from Region 4446 at 12:38 UTC on May 26, while several partly hidden flares were observed on the northeast limb as a new sunspot group rotated onto the visible disk.
A Type II radio emission began at 12:44 UTC on May 26 and was likely associated with the C9.7 flare. The estimated shock speed was 650 km/s (404 mi/s), but any related CME was too faint or narrow to be seen clearly in available coronagraph imagery.
A separate faint, possibly partial halo CME seen from about 22:30 UTC on May 26 was still under analysis.
An S1 storm is the lowest level on NOAA’s solar radiation storm scale and can cause minor high-frequency radio effects in polar regions. Stronger S2 to S5 storms can affect satellites, navigation systems, aviation, and biological exposure risk.
References:
1 Forecast Discussion – NOAA/SWPC – May 26/27, 2026