G4 (Severe) Geomagnetic Storm of October 26, 1991
A G4 (severe) geomagnetic storm began October 26, 1991, reached Kp 8+ and lasted 84 hours, with a daily Ap as high as 128; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.
The Kp through the storm
Began Oct 26, 12:00 UTC, peaked in the block from Oct 28, 15:00 UTC, ended Oct 30, 00:00 UTC.
The storm
- Peak Kp
- 8+ (8.33)
- NOAA level
- G4 Severe
- Duration
- 84 hours
- Highest daily Ap
- 128
- Highest 3-hour ap
- 236
- Rank since 1932
- 77 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 43 degrees geomagnetic latitude: 35°N in the central US, 34°N in the eastern US, 37°N in the western US and 44°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
- San Francisco, CA 43.3° geomag.
- Little Rock, AR 43.3° geomag.
- Oakland, CA 43.4° geomag.
- Modesto, CA 43.4° geomag.
- Huntsville, AL 43.6° geomag.
- Stockton, CA 43.7° geomag.
- Oklahoma City, OK 43.7° geomag.
- New South Memphis, TN 43.8° geomag.
- Memphis, TN 43.9° geomag.
- Santa Rosa, CA 44.0° geomag.
106 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 |
|---|---|---|
| October 26, 1991 | 32 | 5 |
| October 27, 1991 | 50 | 6+ |
| October 28, 1991 | 98 | 8+ |
| October 29, 1991 | 128 | 8+ |
Flares before it
| Peak (UTC) | Class | Region | Before the storm began |
|---|---|---|---|
| Oct 27, 05:49 UTC | X6.1 | NOAA 6891 | during it |
| Oct 24, 02:41 UTC | X2.1 | NOAA 6891 | 57 hours |
| Oct 27, 02:10 UTC | X1.9 | NOAA 6891 | during it |
| Oct 26, 20:51 UTC | X1.7 | NOAA 6891 | during it |
| Oct 24, 22:36 UTC | M9.8 | NOAA 6891 | 37 hours |
| Oct 29, 18:55 UTC | M4.7 | NOAA 6891 | 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, Oct 25The storm after, Oct 30Every storm of 1991The 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 2026-09-29. 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.