G5 (Extreme) Geomagnetic Storm of September 21, 1946
A G5 (extreme) geomagnetic storm began September 21, 1946, reached Kp 9 and lasted 60 hours, with a daily Ap as high as 198; at that Kp the aurora may be seen from about 40 degrees geomagnetic latitude.
The Kp through the storm
Began Sep 21, 15:00 UTC, peaked in the block from Sep 22, 12:00 UTC, ended Sep 24, 03:00 UTC.
The storm
- Peak Kp
- 9 (9.00)
- NOAA level
- G5 Extreme
- Duration
- 60 hours
- Highest daily Ap
- 198
- Highest 3-hour ap
- 400
- Rank since 1932
- 10 of 4,139
How far south the aurora could have been seen
At Kp 9 NOAA’s scales put the aurora’s reach at about 40 degrees geomagnetic latitude: 32°N in the central US, 31°N in the eastern US, 34°N in the western US and 40°N in northern Europe.
NOAA says a G5 storm’s aurora has been seen as low as Florida and southern Texas.
US cities within reach, the southernmost first
- Anaheim, CA 40.0° geomag.
- Corona, CA 40.1° geomag.
- Los Angeles, CA 40.2° geomag.
- Riverside, CA 40.2° geomag.
- Moreno Valley, CA 40.2° geomag.
- Chandler, AZ 40.2° geomag.
- Oxnard, CA 40.2° geomag.
- Hollywood, CA 40.2° geomag.
- Pomona, CA 40.3° geomag.
- Gilbert, AZ 40.3° geomag.
164 US cities of 150,000 people or more sit at or poleward of that line. Low on the northern horizon under a dark, clear sky; not a record of sightings.
Day by day
The daily Ap is the mean of the day’s eight three-hour ap values.
| UT day | Daily Ap | Highest Kp that day |
|---|---|---|
| September 21, 1946 | 22 | 5+ |
| September 22, 1946 | 198 | 9 |
| September 23, 1946 | 146 | 8 |
| September 24, 1946 | 21 | 5+ |
More
The storm before, Sep 17The storm after, Sep 27Every storm of 1946The strongest on recordThis day in weather historyTonight’s aurora forecast
Sources
Kp and ap from GFZ Helmholtz Centre for Geosciences, Geomagnetic Observatory Niemegk, licensed CC BY 4.0: Matzka, J., Stolle, C., Yamazaki, Y., Bronkalla, O. and Morschhauser, A., 2021, The geomagnetic Kp index and derived indices of geomagnetic activity, Space Weather, doi:10.1029/2020SW002641; data doi:10.5880/Kp.0001. Storms, levels and aurora reach worked out by WeatherOverTime from those values and NOAA’s space weather scales. Definitive through August 31, 2026; later values are preliminary. Rebuilt 2026-09-24.