G4 (Severe) Geomagnetic Storm of October 20, 1989

Name: G4 (Severe) Geomagnetic Storm of October 20, 1989
Start: 1989-10-20T09:00:00.000Z
End: 1989-10-23T09:00:00.000Z
Location: Earth

October 20 to 23, 1989 · 72 hours

A G4 (severe) geomagnetic storm began October 20, 1989, reached Kp 8+ and lasted 72 hours, with a daily Ap as high as 146; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.

The Kp through the storm

Began Oct 20, 09:00 UTC, peaked in the block from Oct 20, 18:00 UTC, ended Oct 23, 09:00 UTC.

From 00 UT the day before to the end of the day after. Blocks in the storm are colored by level; the rest are gray.

The storm

Peak Kp: 8+ (8.33)
NOAA level: G4 Severe
Duration: 72 hours
Highest daily Ap: 146
Highest 3-hour ap: 236
Rank since 1932: 75 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.

Tonight’s aurora forecast

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 20, 1989	112	8+
October 21, 1989	146	8+
October 22, 1989	51	6+
October 23, 1989	23	6-

Flares before it

Peak (UTC)	Class	Region	Before the storm began
Oct 19, 12:55 UTC	X13.0	NOAA 5747	20 hours
Oct 22, 17:57 UTC	X2.9	NOAA 5747	during it
Oct 18, 18:25 UTC	M7.4	NOAA 5747	39 hours
Oct 21, 23:56 UTC	M3.1	NOAA 5747	during it
Oct 18, 00:15 UTC	M2.7	NOAA 5747	57 hours
Oct 21, 01:55 UTC	M2.4	NOAA 5747	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.

The storm before, Oct 19 The storm after, Oct 25 Every storm of 1989 The strongest on record This day in weather history Tonight’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 2026-09-26. 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.