G4 storm event · 1940
The June 25, 1940 geomagnetic storm
Answer
3-hour Kp, a day either side
Of the 194 G4 events Glow finds since 1932, this one ranks 71st by peak Kp and 112th 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 49.3 degrees geomagnetic. 1191 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.
| Country | Towns |
|---|---|
| United Kingdom | 328 |
| United States | 246 |
| Canada | 181 |
| Sweden | 97 |
| Russia | 94 |
| Finland | 78 |
| Denmark | 61 |
| Norway | 41 |
| Ireland | 39 |
| Estonia | 7 |
| Iceland | 6 |
| New Zealand | 4 |
| Latvia | 2 |
| Australia | 1 |
| 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
1940 had 4 other G4 periods, and 71 storm days in all. The previous G4 period began on April 25, 1940, and the next on March 1, 1941.
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, 49.3 degrees for this peak, not a map of the night.
Questions
- How strong was the June 25, 1940 geomagnetic storm?
- Kp peaked at 8+ in the GFZ record, with 3 hours at Kp 8 or more.
- Where could aurora be seen?
- SWPC's rule puts the average oval edge near 49.3 degrees geomagnetic at the peak, over 1191 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.