G3 (Strong) Geomagnetic Storm of March 18, 1973

March 18 to 25, 1973 · 171 hours

A G3 (strong) geomagnetic storm began March 18, 1973, reached Kp 7 and lasted 171 hours, with a daily Ap as high as 82; at that Kp the aurora may be seen from about 50 degrees geomagnetic latitude.

The Kp through the storm

Began Mar 18, 21:00 UTC, peaked in the block from Mar 19, 18:00 UTC, ended Mar 26, 00:00 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: 7 (7.00)
NOAA level: G3 Strong
Duration: 171 hours
Highest daily Ap: 82
Highest 3-hour ap: 132
Rank since 1932: 396 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 50 degrees geomagnetic latitude: 42°N in the central US, 41°N in the eastern US, 44°N in the western US and 51°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

Washington Heights, NY 50.0° geomag.
The Bronx, NY 50.0° geomag.
Fort Wayne, IN 50.1° geomag.
Des Moines, IA 50.1° geomag.
Yonkers, NY 50.1° geomag.
Akron, OH 50.2° geomag.
Salem, OR 50.3° geomag.
Aurora, IL 50.6° geomag.
Cleveland, OH 50.6° geomag.
Toledo, OH 50.7° geomag.

32 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
March 18, 1973	20	5
March 19, 1973	82	7
March 20, 1973	80	7-
March 21, 1973	58	6
March 22, 1973	53	6
March 23, 1973	50	6-
March 24, 1973	49	6-
March 25, 1973	50	5+

The storm before, Mar 6 The storm after, Mar 26 Every storm of 1973 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.