G4 (Severe) Geomagnetic Storm of September 18, 1989
A G4 (severe) geomagnetic storm began September 18, 1989, reached Kp 8 and lasted 21 hours, with a daily Ap as high as 70; at that Kp the aurora may be seen from about 45 degrees geomagnetic latitude.
The Kp through the storm
Began Sep 18, 15:00 UTC, peaked in the block from Sep 19, 03:00 UTC, ended Sep 19, 12:00 UTC.
The storm
- Peak Kp
- 8 (8.00)
- NOAA level
- G4 Severe
- Duration
- 21 hours
- Highest daily Ap
- 70
- Highest 3-hour ap
- 207
- Rank since 1932
- 167 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 45 degrees geomagnetic latitude: 37°N in the central US, 36°N in the eastern US, 39°N in the western US and 45°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
- Nashville, TN 45.1° geomag.
- Durham, NC 45.1° geomag.
- Greensboro, NC 45.2° geomag.
- Winston-Salem, NC 45.2° geomag.
- Clarksville, TN 45.4° geomag.
- Reno, NV 45.4° geomag.
- Springfield, MO 45.7° geomag.
- Wichita, KS 45.9° geomag.
- Chesapeake, VA 46.0° geomag.
- Norfolk, VA 46.0° geomag.
84 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 18, 1989 | 52 | 8- |
| September 19, 1989 | 70 | 8 |
Flares before it
| Peak (UTC) | Class | Region | Before the storm began |
|---|---|---|---|
| Sep 15, 23:48 UTC | M2.3 | NOAA 5687 | 63 hours |
| Sep 16, 16:34 UTC | M1.2 | NOAA 5683 | 46 hours |
| Sep 17, 18:48 UTC | M1.2 | NOAA 5686 | 20 hours |
| Sep 19, 10:03 UTC | M1.2 | NOAA 5698 | during it |
| Sep 18, 18:58 UTC | M1.1 | NOAA 5686 | during it |
The strongest M and X class flares GOES recorded from four days before the storm began to its end, the window in which a coronal mass ejection launched with a flare usually arrives. A flare in this list is context, not proof of the cause: many storms come from coronal holes, and many flares launch nothing toward Earth.
More
The storm before, Sep 15The storm after, Sep 26Every storm of 1989The 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 2026-09-25. Flares from NOAA NCEI’s GOES X-ray flare reports (1975 to 2017) and SWPC’s edited solar event lists, public domain; classes as NOAA recorded them at the time.