G1 – Minor Geomagnetic Storm Alert issued on December 21

A G1 – Minor Geomagnetic Storm Alert was issued for December 21, 2025. The alert indicates that the geomagnetic K-index reached 5 at 23:14 UTC, and is valid for the synoptic period between 21:00 and 24:00 UTC. The alert was issued by NOAA’s Space Weather Prediction Center at 23:16 UTC.

The area of impact is expected to be primarily poleward of 60 degrees geomagnetic latitude. Potential impacts include weak power grid fluctuations, minor impact on satellite operations, and the possible visibility of auroras at high latitudes, including the northern tier of the United States such as northern Michigan and Maine.

This event was triggered by enhanced solar wind conditions associated with the onset of Corotating Interaction Region (CIR) effects. During this period, the solar wind speed increased from around 450 km/s to a peak near 650 km/s, and the geomagnetic field reached active levels in response.