G1 (Minor) Geomagnetic Storm of July 31, 1985
A G1 (minor) geomagnetic storm began July 31, 1985, reached Kp 5+ and lasted 15 hours, with a daily Ap as high as 36; at that Kp the aurora may be seen from about 58 degrees geomagnetic latitude.
The Kp through the storm
Began Jul 31, 09:00 UTC, peaked in the block from Jul 31, 09:00 UTC, ended Aug 1, 00:00 UTC.
The storm
- Peak Kp
- 5+ (5.33)
- NOAA level
- G1 Minor
- Duration
- 15 hours
- Highest daily Ap
- 36
- Highest 3-hour ap
- 56
- Rank since 1932
- 2,210 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 |
|---|---|---|
| July 31, 1985 | 36 | 5+ |
Flares before it
GOES recorded no M or X class flare from four days before this storm to its end. Storms without a large flare are commonly driven by fast solar wind from a coronal hole, or by an eruption no flare announced.
More
The storm before, Jul 17The storm after, Aug 12Every storm of 1985The 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. 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.