G3 (Strong) Geomagnetic Storm of October 17, 1970
A G3 (strong) geomagnetic storm began October 17, 1970, reached Kp 7- and lasted 15 hours, with a daily Ap as high as 39; at that Kp the aurora may be seen from about 52 degrees geomagnetic latitude.
The Kp through the storm
Began Oct 17, 15:00 UTC, peaked in the block from Oct 18, 03:00 UTC, ended Oct 18, 06:00 UTC.
The storm
- Peak Kp
- 7- (6.67)
- NOAA level
- G3 Strong
- Duration
- 15 hours
- Highest daily Ap
- 39
- Highest 3-hour ap
- 111
- Rank since 1932
- 671 of 4,139
Time at each level
Worked out by WeatherOverTime from the Kp record and NOAA’s scale.
| Level | Hours at that level |
|---|---|
| G1 Minor | 6 |
| G2 Moderate | 6 |
| G3 Strong | 3 |
Whole three-hour blocks of GFZ’s Kp at that level, counted from Kp 5. The peak block began 12 hours after the first storm block.
How far south the aurora could have been seen
At Kp 7- NOAA’s scales put the aurora’s reach at about 52 degrees geomagnetic latitude: 43°N in the central US, 42°N in the eastern US, 46°N in the western US and 52°N in northern Europe.
NOAA says a G3 storm’s aurora has been seen as low as Illinois and Oregon.
US cities within reach, the southernmost first
- Sioux Falls, SD 51.8° geomag.
- Madison, WI 51.8° geomag.
- Milwaukee, WI 51.9° geomag.
- Grand Rapids, MI 51.9° geomag.
- Buffalo, NY 52.0° geomag.
- Tri-Cities, WA 52.1° geomag.
- Rochester, NY 52.3° geomag.
- Tacoma, WA 52.7° geomag.
- Seattle, WA 53.0° geomag.
- Saint Paul, MN 53.5° geomag.
13 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 |
|---|---|---|
| October 17, 1970 | 34 | 6 |
| October 18, 1970 | 39 | 7- |
More
The storm before, Oct 16The storm after, Oct 23Every 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 2026-10-01.