ALERT: Geomagnetic K-index of 4 Alert issued on October 18

An ALERT: Geomagnetic K-index of 4 was issued by NOAA SWPC at 02:42 UTC on October 18, 2025. The alert is valid for the synoptic period from 0000 to 0300 UTC on the same date, indicating that geomagnetic activity has reached a K-index of 4.

Potential impacts from this alert are primarily expected poleward of 65 degrees geomagnetic latitude. Weak power grid fluctuations can occur due to induced currents. Additionally, aurora may be visible at high latitudes such as Canada and Alaska. These effects are consistent with geomagnetic activity at this level.

According to the latest SWPC Forecast Discussion, this period of elevated geomagnetic activity may be associated with glancing effects from coronal mass ejections (CMEs) that left the Sun during October 13–15, 2025. No major Earth-directed CMEs have been observed, and the bulk of the material is expected to pass ahead of Earth’s orbit. The geomagnetic field is forecasted to remain unsettled to active on October 18, with a chance for additional disturbances into the early portions of October 19 as a negative polarity coronal hole high speed stream (CH HSS) influences the solar wind environment.