A strong G3 geomagnetic storm hit Earth on Oct. 10, 2026, according to alerts from NOAA’s Space Weather Prediction Center. The agency’s feed shows a K-index of 7 reached at 1926 UTC, the level that defines a G3 on NOAA’s five-step geomagnetic storm scale, after an earlier alert for the same threshold at 1758 UTC and a warning issued at 1742 UTC.
The storm arrived on a busy day for the Sun. A moderate M6.4 solar flare peaked less than three hours before the first G3 alert, and the planetary index had already climbed through K5 and K6 levels before it reached K7. Forecasts beforehand had pointed to a weaker storm.
The alerts that marked the G3 threshold
The Space Weather Prediction Center posts its bulletins as a machine-readable feed, and the alert product feed for Oct. 10 reads like a staircase. Alert K05A recorded K5, a G1 level, at 1535 UTC. Alert K06A recorded K6, a G2 level, at 1718 UTC and again at 1837 UTC. Warning WARK07, issued at 1742 UTC and valid until 2359 UTC, flagged the chance of K7 or greater. Alert ALTK07 serial 219 then reported K7 at 1758 UTC, and serial 220 reported it at 1926 UTC, with the line “Threshold Reached: 2026 Oct 10 1926 UTC” and the label “Noaa Scale: G3 – Strong.”
The same feed shows the index dropping back afterward. Alert K05A, G1, was logged again at 2145 UTC, and an extended warning for K4 ran through Oct. 11 at 1200 UTC. A K4 warning returned on Oct. 13 and was extended until 1200 UTC on Oct. 14. No G3 entries appear after Oct. 10 in the entries read for this article, and the most recent entry in the feed was posted on Oct. 13 at 2058 UTC.
An M6.4 flare and a shock before the storm
The day’s solar activity is also recorded in the feed. An M6.4 flare at N08W50 began at 1458 UTC, peaked at 1510 UTC and ended at 1520 UTC, and SWPC rated it R2, a moderate radio blackout. On NOAA’s space weather scales, an R2 event means a limited blackout of high-frequency radio on the sunlit side of Earth, with contact lost for tens of minutes, and it corresponds to an M5 X-ray flux level. NOAA says about 350 such events occur per solar cycle.
Other entries show the disturbance arriving. A warning at 1445 UTC flagged a sudden geomagnetic impulse, with a shock at 1443 UTC, and a summary logged a 56 nanotesla impulse at station NGK at 1516 UTC. A Type II radio emission, a signature of a shock moving out through the corona, began at 1515 UTC with an estimated speed of 929 kilometers per second. The feed does not link those entries to a particular eruption, so the exact origin of the Oct. 10 storm is not stated there.
Sky at Night Magazine content editor Iain Todd reported on Oct. 8 that a coronal mass ejection erupted from the Sun at 0610 UTC on Oct. 6 and that the UK Met Office was watching for a glancing blow from another ejection on Saturday, Oct. 10, while NOAA had reported a G2 storm outlook. His aurora forecast listed Washington, Idaho, Montana, North Dakota, South Dakota, Minnesota, Wisconsin, Michigan, Maine, Vermont and New Hampshire as likely viewing states, with Oregon, Wyoming, Nebraska, Iowa, Illinois, Indiana, Ohio, Colorado, Pennsylvania and New York listed as possible. The magazine also noted that the Sun is past the peak of its 11-year cycle, which it credited for the recent rise in aurora frequency and strength.
What a G3 means for power, satellites and the sky
NOAA’s scale describes a G3 in practical terms. Power systems may need voltage adjustments and some protective devices may trip false alarms. Satellites can pick up surface charge and low-orbit spacecraft can feel added drag. Low-frequency navigation can be disrupted intermittently, high-frequency radio can be spotty, and aurora may be visible as far south as Illinois and Oregon. NOAA lists about 200 G3 events per 11-year solar cycle, roughly 130 days.
The agency’s explainer on geomagnetic storms adds that the largest storms come from coronal mass ejections carrying about a billion tons of plasma, and that storms induce currents in power grids and pipelines and can disrupt satellite navigation. The agency’s watches, warnings and alerts are listed on its bulletin page, which points readers to the same JSON file used here.
The forecast side of the story shifted during the week. A G2 watch was issued on Oct. 9 and was then revised to carry G1 for Oct. 10 and G2 for Oct. 11. On Oct. 11 at 1702 UTC SWPC canceled the G2 watch for that day, and at 1911 UTC it issued a G1 watch for Oct. 13, with conditions below G1 expected on Oct. 12 and 14. The G3 on Oct. 10 therefore came in above the highest level any of those watches had named for that day.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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