G3 (Strong) Geomagnetic Storm of March 21, 1991
A G3 (strong) geomagnetic storm began March 21, 1991, reached Kp 7- and lasted 3 hours, with a daily Ap as high as 27; at that Kp the aurora may be seen from about 52 degrees geomagnetic latitude.
The Kp through the storm
Began Mar 21, 12:00 UTC, peaked in the block from Mar 21, 12:00 UTC, ended Mar 21, 15:00 UTC.
The storm
- Peak Kp
- 7- (6.67)
- NOAA level
- G3 Strong
- Duration
- 3 hours
- Highest daily Ap
- 27
- Highest 3-hour ap
- 111
- Rank since 1932
- 767 of 4,139
How far south the aurora could have been seen
At Kp 7- NOAA’s scales put the aurora’s reach at about 52 degrees geomagnetic latitude: 43°N in the central US, 42°N in the eastern US, 46°N in the western US and 52°N in northern Europe.
NOAA says a G3 storm’s aurora has been seen as low as Illinois and Oregon.
US cities within reach, the southernmost first
- Sioux Falls, SD 51.8° geomag.
- Madison, WI 51.8° geomag.
- Milwaukee, WI 51.9° geomag.
- Grand Rapids, MI 51.9° geomag.
- Buffalo, NY 52.0° geomag.
- Tri-Cities, WA 52.1° geomag.
- Rochester, NY 52.3° geomag.
- Tacoma, WA 52.7° geomag.
- Seattle, WA 53.0° geomag.
- Saint Paul, MN 53.5° geomag.
13 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 |
|---|---|---|
| March 21, 1991 | 27 | 7- |
Flares before it
| Peak (UTC) | Class | Region | Before the storm began |
|---|---|---|---|
| Mar 17, 21:26 UTC | X1.0 | NOAA 6545 | 87 hours |
| Mar 20, 06:16 UTC | M7.0 | NOAA 6555 | 30 hours |
| Mar 19, 01:58 UTC | M6.7 | NOAA 6545 | 58 hours |
| Mar 18, 17:31 UTC | M5.3 | NOAA 6545 | 66 hours |
| Mar 19, 01:22 UTC | M3.6 | 59 hours | |
| Mar 21, 08:18 UTC | M2.7 | 4 hours |
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, Mar 13The storm after, Mar 24Every 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 2026-09-24. 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.