G4 (Severe) Geomagnetic Storm of April 21, 1970
A G4 (severe) geomagnetic storm began April 21, 1970, reached Kp 8+ and lasted 27 hours, with a daily Ap as high as 90; at that Kp the aurora may be seen from about 43 degrees geomagnetic latitude.
The Kp through the storm
Began Apr 21, 06:00 UTC, peaked in the block from Apr 21, 18:00 UTC, ended Apr 22, 09:00 UTC.
The storm
- Peak Kp
- 8+ (8.33)
- NOAA level
- G4 Severe
- Duration
- 27 hours
- Highest daily Ap
- 90
- Highest 3-hour ap
- 236
- Rank since 1932
- 113 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.
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 |
|---|---|---|
| April 21, 1970 | 90 | 8+ |
| April 22, 1970 | 41 | 7 |
More
The storm before, Apr 19The storm after, May 27Every storm of 1970The strongest on recordThis day in weather historyTonight’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.