K-index of 4 – Geomagnetic Storm Alert issued on December 21

A Geomagnetic K-index of 4 Alert has been issued by NOAA SWPC for December 21, 2025, after the threshold was reached at 03:00 UTC. The active alert covers the synoptic period from 03:00 to 06:00 UTC and was officially released at 03:04 UTC.

According to NOAA, the area of impact is expected mainly poleward of 65 degrees geomagnetic latitude. Potential impacts include weak power grid fluctuations caused by induced currents, primarily affecting those regions. Additionally, auroras may be visible at high latitudes such as Canada and Alaska. These effects are consistent with what is typically seen during periods of increased geomagnetic activity at this K-index level.

The current alert coincides with a period of waning negative polarity coronal hole high-speed stream influences and the possible beginning of a solar sector boundary crossing, as noted in the latest forecast discussion. No recent significant CME or solar flare activity has been identified as a direct cause of this alert, and solar wind conditions are generally expected to remain ambient on December 21.