G4 (Severe) Geomagnetic Storm of May 15, 2005

Name: G4 (Severe) Geomagnetic Storm of May 15, 2005
Start: 2005-05-15T00:00:00.000Z
End: 2005-05-16T15:00:00.000Z
Location: Earth

May 15 to 16, 2005 · 39 hours

A G4 (severe) geomagnetic storm began May 15, 2005, reached Kp 8+ and lasted 39 hours, with a daily Ap as high as 87; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.

The Kp through the storm

Began May 15, 00:00 UTC, peaked in the block from May 15, 06:00 UTC, ended May 16, 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+ (8.33)
NOAA level: G4 Severe
Duration: 39 hours
Highest daily Ap: 87
Highest 3-hour ap: 236
Rank since 1932: 108 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
May 15, 2005	87	8+
May 16, 2005	41	6+

Flares before it

Peak (UTC)	Class	Region	Before the storm began
May 13, 16:57 UTC	M8.0	NOAA 10759	31 hours
May 15, 22:36 UTC	M3.5	NOAA 10763	during it
May 12, 07:33 UTC	M1.6	NOAA 10759	64 hours
May 16, 09:08 UTC	M1.6	NOAA 10763	during it
May 12, 17:41 UTC	M1.4	NOAA 10759	54 hours
May 16, 02:43 UTC	M1.4	NOAA 10763	during it

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, May 8 The storm after, May 20 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.