G4 (Severe) Geomagnetic Storm of June 12, 1990

Name: G4 (Severe) Geomagnetic Storm of June 12, 1990
Start: 1990-06-12T12:00:00.000Z
End: 1990-06-14T18:00:00.000Z
Location: Earth

June 12 to 14, 1990 · 54 hours

A G4 (severe) geomagnetic storm began June 12, 1990, reached Kp 8+ and lasted 54 hours, with a daily Ap as high as 89; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.

The Kp through the storm

Began Jun 12, 12:00 UTC, peaked in the block from Jun 12, 21:00 UTC, ended Jun 14, 18:00 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: 54 hours
Highest daily Ap: 89
Highest 3-hour ap: 236
Rank since 1932: 88 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
June 12, 1990	89	8+
June 13, 1990	70	8-
June 14, 1990	79	8-

Flares before it

Peak (UTC)	Class	Region	Before the storm began
Jun 12, 05:29 UTC	M6.4	NOAA 6089	7 hours
Jun 11, 09:48 UTC	M4.5	NOAA 6089	26 hours
Jun 10, 15:06 UTC	M3.3	NOAA 6089	45 hours
Jun 9, 16:48 UTC	M2.5	NOAA 6089	67 hours
Jun 10, 07:27 UTC	M2.3	NOAA 6089	53 hours
Jun 10, 22:10 UTC	M1.7	NOAA 6089	38 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.

The storm before, Jun 9 The storm after, Jul 26 Every storm of 1990 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 2026-09-24. 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.