Geomagnetic K-index of 4 Alert issued on October 18

A Geomagnetic K-index of 4 Alert was issued on October 18, 2025, by the NOAA Space Weather Prediction Center. The alert is valid for the synoptic period from 15:00 to 18:00 UTC, during which the geomagnetic activity reached the threshold indicating active conditions.

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. Additionally, auroras may be visible at high latitudes such as Canada and Alaska. These effects are generally observed during elevated geomagnetic activity and reflect increased interactions between the solar wind and Earth’s magnetic field.

The triggering event for this elevated geomagnetic activity is associated with glancing influences from coronal mass ejections (CMEs) that left the Sun between October 13 and 15, 2025. Forecasts suggest that while the majority of CME material will pass ahead of Earth’s orbit, glancing blows may result in unsettled to active geomagnetic conditions on October 18. Further unsettled to active periods are anticipated as Earth transitions into a negative polarity coronal hole high speed stream (CH HSS) beginning October 19, 2025.