G2 (Moderate) Geomagnetic Storm of September 13, 1947

September 13 to 15, 1947 · 72 hours

A G2 (moderate) geomagnetic storm began September 13, 1947, reached Kp 6+ and lasted 72 hours, with a daily Ap as high as 62; at that Kp the aurora may be seen from about 53 degrees geomagnetic latitude.

The Kp through the storm

Began Sep 13, 00:00 UTC, peaked in the block from Sep 14, 15:00 UTC, ended Sep 16, 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: 6+ (6.33)
NOAA level: G2 Moderate
Duration: 72 hours
Highest daily Ap: 62
Highest 3-hour ap: 94
Rank since 1932: 773 of 4,139

How far south the aurora could have been seen

At Kp 6+ NOAA’s scales put the aurora’s reach at about 53 degrees geomagnetic latitude: 45°N in the central US, 44°N in the eastern US, 48°N in the western US and 54°N in northern Europe.

NOAA says a G2 storm’s aurora has been seen as low as New York and Idaho.

US cities within reach, the southernmost first

Saint Paul, MN 53.5° geomag.
Minneapolis, MN 53.5° geomag.
Spokane, WA 53.7° geomag.
Anchorage, AK 61.9° geomag.

4 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 13, 1947	48	6-
September 14, 1947	59	6+
September 15, 1947	62	6+

The storm before, Sep 11 The storm after, Sep 16 Every storm of 1947 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.