K-index of 4 – Geomagnetic K-index Alert issued on December 17

A Geomagnetic K-index Alert was issued by the NOAA SWPC for a K-index of 4 on December 17, 2025. The alert was released at 20:50 UTC and is active for the synoptic period from 18:00 to 21:00 UTC.

According to the alert, the area of impact is primarily poleward of 65 degrees geomagnetic latitude. Potential impacts include weak power grid fluctuations due to induced currents. Auroras may be visible at high latitudes such as those in Canada and Alaska.

The forecast discussion from NOAA SWPC indicates that the geomagnetic activity is related to the influence of a negative polarity coronal hole high-speed solar wind stream (CH HSS). Solar wind speeds increased significantly, and geomagnetic activity is expected to continue at quiet to active levels through December 18 as HSS activity persists.