G4 (Severe) Geomagnetic Storm of September 9, 1973
A G4 (severe) geomagnetic storm began September 9, 1973, reached Kp 8- and lasted 6 hours, with a daily Ap as high as 44; at that Kp the aurora may be seen from about 47 degrees geomagnetic latitude.
The Kp through the storm
Began Sep 9, 18:00 UTC, peaked in the block from Sep 9, 21:00 UTC, ended Sep 10, 00:00 UTC.
The storm
- Peak Kp
- 8- (7.67)
- NOAA level
- G4 Severe
- Duration
- 6 hours
- Highest daily Ap
- 44
- Highest 3-hour ap
- 179
- Rank since 1932
- 286 of 4,139
How far south the aurora could have been seen
At Kp 8- NOAA’s scales put the aurora’s reach at about 47 degrees geomagnetic latitude: 38°N in the central US, 37°N in the eastern US, 41°N in the western US and 47°N in northern Europe.
NOAA says a G4 storm’s aurora has been seen as low as Alabama and northern California.
US cities within reach, the southernmost first
- Richmond, VA 46.7° geomag.
- Lexington, KY 47.0° geomag.
- Lexington-Fayette, KY 47.0° geomag.
- Louisville, KY 47.2° geomag.
- Lakewood, CO 47.3° geomag.
- Denver, CO 47.3° geomag.
- Aurora, CO 47.3° geomag.
- St. Louis, MO 47.3° geomag.
- Overland Park, KS 47.4° geomag.
- Meads, KY 47.5° geomag.
71 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 9, 1973 | 44 | 8- |
More
The storm before, Aug 28The storm after, Sep 10Every storm of 1973The 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.