G1 (Minor) Geomagnetic Storm of April 10, 1975
A G1 (minor) geomagnetic storm began April 10, 1975, reached Kp 5+ and lasted 18 hours, with a daily Ap as high as 35; at that Kp the aurora may be seen from about 58 degrees geomagnetic latitude.
The Kp through the storm
Began Apr 10, 21:00 UTC, peaked in the block from Apr 11, 00:00 UTC, ended Apr 11, 15:00 UTC.
The storm
- Peak Kp
- 5+ (5.33)
- NOAA level
- G1 Minor
- Duration
- 18 hours
- Highest daily Ap
- 35
- Highest 3-hour ap
- 56
- Rank since 1932
- 2,188 of 4,139
How far south the aurora could have been seen
At Kp 5+ NOAA’s scales put the aurora’s reach at about 58 degrees geomagnetic latitude: 50°N in the central US, 49°N in the eastern US, 53°N in the western US and 59°N in northern Europe.
NOAA says a G1 storm’s aurora commonly visible at high latitudes: northern Michigan and Maine.
US cities within reach, the southernmost first
- Anchorage, AK 61.9° geomag.
1 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 10, 1975 | 35 | 5+ |
| April 11, 1975 | 34 | 5+ |
More
The storm before, Apr 8The storm after, Apr 12Every storm of 1975The 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.