G4 (Severe) Geomagnetic Storm of September 10, 2005

Name: G4 (Severe) Geomagnetic Storm of September 10, 2005
Start: 2005-09-10T15:00:00.000Z
End: 2005-09-13T15:00:00.000Z
Location: Earth

September 10 to 13, 2005 · 72 hours

A G4 (severe) geomagnetic storm began September 10, 2005, reached Kp 8- and lasted 72 hours, with a daily Ap as high as 101; at that Kp the aurora may be seen from about 47 degrees geomagnetic latitude.

The Kp through the storm

Began Sep 10, 15:00 UTC, peaked in the block from Sep 11, 06:00 UTC, ended Sep 13, 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: 72 hours
Highest daily Ap: 101
Highest 3-hour ap: 179
Rank since 1932: 195 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 10, 2005	33	6-
September 11, 2005	101	8-
September 12, 2005	75	7
September 13, 2005	44	6

Flares before it

Peak (UTC)	Class	Region	Before the storm began
Sep 7, 17:40 UTC	X17.0	NOAA 10808	69 hours
Sep 9, 20:04 UTC	X6.2	NOAA 10808	19 hours
Sep 8, 21:06 UTC	X5.4	NOAA 10808	42 hours
Sep 9, 09:59 UTC	X3.6	NOAA 10808	29 hours
Sep 10, 22:11 UTC	X2.1	NOAA 10808	during it
Sep 9, 03:00 UTC	X1.1	NOAA 10808	36 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 2 The storm after, Sep 15 Every storm of 2005 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.