G2 (Moderate) Geomagnetic Storm of July 21, 1991
A G2 (moderate) geomagnetic storm began July 21, 1991, reached Kp 6- and lasted 3 hours, with a daily Ap as high as 32; at that Kp the aurora may be seen from about 57 degrees geomagnetic latitude.
The Kp through the storm
Began Jul 21, 03:00 UTC, peaked in the block from Jul 21, 03:00 UTC, ended Jul 21, 06:00 UTC.
The storm
- Peak Kp
- 6- (5.67)
- NOAA level
- G2 Moderate
- Duration
- 3 hours
- Highest daily Ap
- 32
- Highest 3-hour ap
- 67
- Rank since 1932
- 2,093 of 4,139
How far south the aurora could have been seen
At Kp 6- NOAA’s scales put the aurora’s reach at about 57 degrees geomagnetic latitude: 48°N in the central US, 47°N in the eastern US, 51°N in the western US and 57°N in northern Europe.
NOAA says a G2 storm’s aurora has been seen as low as New York and Idaho.
US cities within reach, the southernmost first
- Anchorage, AK 61.9° geomag.
1 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 |
|---|---|---|
| July 21, 1991 | 32 | 6- |
Flares before it
| Peak (UTC) | Class | Region | Before the storm began |
|---|---|---|---|
| Jul 17, 06:32 UTC | X1.1 | NOAA 6729 | 92 hours |
| Jul 18, 14:28 UTC | X1.0 | NOAA 6734 | 61 hours |
| Jul 20, 01:42 UTC | X1.0 | NOAA 6734 | 25 hours |
| Jul 17, 21:24 UTC | M3.0 | NOAA 6728 | 78 hours |
| Jul 19, 17:25 UTC | M2.0 | NOAA 6737 | 34 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, Jul 19The storm after, Aug 1Every 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.