How strong was the September 18, 1941 geomagnetic storm?

Kp peaked at 9 in the GFZ record, with 27 hours at Kp 8 or more.

Where could aurora be seen?

SWPC's rule puts the average oval edge near 48.0 degrees geomagnetic at the peak, over 1196 of Glow's 1218 towns. Kp alone cannot confirm sightings.

Why does the event start and end where it does?

Glow joins 3-hour blocks at Kp 8 or more that start within 24 hours of each other. The event runs from the first such block to the end of the last.

G4 storm event · 1941

The September 18, 1941 geomagnetic storm

Answer

The GFZ Potsdam record shows Kp at 8 or more for 27 hours between Sep 18, 1941, 06:00 UTC and Sep 19, 1941, 09:00 UTC, peaking at Kp 9, G5 Extreme on NOAA's scale. 3 of those hours reached Kp 9, the G5 level.

Kp 9

27 h at G4 or more

GFZ Kp, Sep 18, 1941, 06:00 UTC to Sep 19, 1941, 09:00 UTC

3-hour Kp, a day either side

Each bar is one 3-hour block, UTC. Ember bars reached Kp 5 or more.

Of the 194 G4 events Glow finds since 1932, this one ranks 1st by peak Kp and 1st by hours at G4 or more. Ties share a rank.

Where the oval edge reached

At the peak, SWPC's rule of thumb puts the average oval edge at 48.0 degrees geomagnetic. 1196 of Glow's 1218 towns sit at or poleward of that line, including 328 in United Kingdom, 246 in United States, 181 in Canada. Of Glow's 10 featured towns, Buffalo, Portland, Maine, Seattle, Minneapolis, Duluth, Anchorage, Fairbanks, Tromsø, Yellowknife, Reykjavík sat at or poleward of it.

Glow towns at or poleward of 48.0 degrees geomagnetic, by country
Country	Towns
United Kingdom	328
United States	246
Canada	181
Sweden	97
Russia	94
Finland	78
Denmark	61
Norway	41
Ireland	39
Estonia	7
New Zealand	7
Iceland	6
Australia	3
Latvia	2
Aland Islands	1
Faroe Islands	1
Greenland	1
Isle of Man	1
Saint Pierre and Miquelon	1
Svalbard and Jan Mayen	1

Before and after

1941 had 3 other G4 periods, and 83 storm days in all. The previous G4 period began on August 4, 1941, and the next on March 1, 1942.

Kp is a planetary index from 13 observatories, recorded in 3-hour blocks, so it cannot say where aurora was seen or for how long. The oval edge here is SWPC's average rule, 48.0 degrees for this peak, not a map of the night.

Questions

How strong was the September 18, 1941 geomagnetic storm?: Kp peaked at 9 in the GFZ record, with 27 hours at Kp 8 or more.
Where could aurora be seen?: SWPC's rule puts the average oval edge near 48.0 degrees geomagnetic at the peak, over 1196 of Glow's 1218 towns. Kp alone cannot confirm sightings.
Why does the event start and end where it does?: Glow joins 3-hour blocks at Kp 8 or more that start within 24 hours of each other. The event runs from the first such block to the end of the last.

Sources and verification

GFZ Potsdam, Kp index data (CC BY 4.0)https://kp.gfz.de/en/data
Matzka et al. 2021, Geomagnetic Kp indexhttps://doi.org/10.5880/Kp.0001
CC BY 4.0https://creativecommons.org/licenses/by/4.0/
NOAA SWPC, Tips on viewing the aurorahttps://www.spaceweather.gov/content/tips-viewing-aurora

Quoted figures last checked against their sources on 2026-09-30; next review 2026-12-31. Computed figures come from SunCalc and the IGRF-14 dipole as described in the methodology. Every quote is listed in the sources ledger.