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

A Geomagnetic K-index of 4 Alert was issued on October 21, 2026, by the NOAA Space Weather Prediction Center. The alert was triggered at 04:29 UTC, indicating that geomagnetic activity reached the threshold of K-index 4 during the 03:00–06:00 UTC synoptic period. The alert is currently active.

Possible impacts from this level of geomagnetic activity primarily affect areas poleward of 65 degrees geomagnetic latitude. Weak power grid fluctuations can occur, and auroras may be visible at high latitudes, such as in Canada and Alaska. These effects are characteristic of active geomagnetic conditions and are most relevant to high-latitude observers and infrastructure.

According to the latest forecast discussion, the geomagnetic field has been influenced by the arrival of a coronal hole high-speed stream (CH HSS). This has enhanced solar wind parameters and led to unsettled to active geomagnetic periods, with G1 (Minor) geomagnetic storming likely during the first half of October 21, 2026, as these effects became geoeffective.