G3 (Strong) Geomagnetic Storm of September 30, 2001

Name: G3 (Strong) Geomagnetic Storm of September 30, 2001
Start: 2001-09-30T15:00:00.000Z
End: 2001-10-04T00:00:00.000Z
Location: Earth

September 30 to October 3, 2001 · 81 hours

A G3 (strong) geomagnetic storm began September 30, 2001, reached Kp 7 and lasted 81 hours, with a daily Ap as high as 69; at that Kp the aurora may be seen from about 50 degrees geomagnetic latitude.

The Kp through the storm

Began Sep 30, 15:00 UTC, peaked in the block from Oct 3, 09:00 UTC, ended Oct 4, 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: 81 hours
Highest daily Ap: 69
Highest 3-hour ap: 132
Rank since 1932: 403 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
September 30, 2001	29	6-
October 1, 2001	48	6
October 2, 2001	52	7-
October 3, 2001	69	7

Flares before it

Peak (UTC)	Class	Region	Before the storm began
Oct 1, 05:15 UTC	M9.1		during it
Sep 28, 08:30 UTC	M3.3	NOAA 9636	55 hours
Sep 28, 10:14 UTC	M2.4	NOAA 9628	53 hours
Sep 29, 11:06 UTC	M1.8	NOAA 9636	28 hours
Oct 2, 10:21 UTC	M1.4		during it
Sep 28, 21:25 UTC	M1.2	NOAA 9634	42 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, Sep 29 The storm after, Oct 11 Every storm of 2001 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 2026-09-25. 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.