G1 – Minor Geomagnetic Storm Alert issued on December 18

A G1 – Minor Geomagnetic Storm Alert was issued on December 18, 2025, after the geomagnetic K-index reached a value of 5. The threshold was reached at 23:30 UTC on December 17, and the alert remains active during the synoptic period from 21:00 to 24:00 UTC.

According to the alert, affected areas are primarily those poleward of 60 degrees geomagnetic latitude. Weak power grid fluctuations may occur due to induced currents, and minor impacts on satellite operations are possible. In addition, auroras may be visible at high latitudes, including areas such as northern Michigan and Maine in the United States.

Forecast discussions from the NOAA Space Weather Prediction Center indicate the geomagnetic activity is being driven by the influence of a negative polarity coronal hole high-speed stream (CH HSS). Solar wind speeds increased to between 550 and 700 km/s, contributing to the elevated geomagnetic activity levels. No Earth-directed coronal mass ejections (CMEs) were observed, and solar activity otherwise remained at low levels.