G4 (Severe) Geomagnetic Storm of September 19, 1951

September 19 to 24, 1951 · 123 hours

A G4 (severe) geomagnetic storm began September 19, 1951, reached Kp 8- and lasted 123 hours, with a daily Ap as high as 106; at that Kp the aurora may be seen from about 47 degrees geomagnetic latitude.

The Kp through the storm

Began Sep 19, 12:00 UTC, peaked in the block from Sep 19, 15:00 UTC, ended Sep 24, 15:00 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- (7.67)
NOAA level: G4 Severe
Duration: 123 hours
Highest daily Ap: 106
Highest 3-hour ap: 179
Rank since 1932: 194 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 47 degrees geomagnetic latitude: 38°N in the central US, 37°N in the eastern US, 41°N in the western US and 47°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

Richmond, VA 46.7° geomag.
Lexington, KY 47.0° geomag.
Lexington-Fayette, KY 47.0° geomag.
Louisville, KY 47.2° geomag.
Lakewood, CO 47.3° geomag.
Denver, CO 47.3° geomag.
Aurora, CO 47.3° geomag.
St. Louis, MO 47.3° geomag.
Overland Park, KS 47.4° geomag.
Meads, KY 47.5° geomag.

71 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
September 19, 1951	59	8-
September 20, 1951	106	7+
September 21, 1951	89	7
September 22, 1951	85	7
September 23, 1951	51	6
September 24, 1951	42	6-

The storm before, Sep 18 The storm after, Sep 25 Every storm of 1951 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.