How Geomagnetic Storms Are Classified (G1โG5)
NOAA classifies geomagnetic storms on a five-step G-scale mirroring the hurricane and Enhanced Fujita scales. Each step corresponds to a Kp threshold and a set of expected impacts.
The Five Levels
G1 Minor (Kp 5): weak power grid fluctuations, aurora visible from high latitudes. G2 Moderate (Kp 6): high-latitude aurora extends toward mid-latitudes, HF radio fades at high latitudes. G3 Strong (Kp 7): mid-latitude aurora, GPS degradation, corrective actions required at some power grids. G4 Severe (Kp 8): widespread aurora visible from lower mid-latitudes, satellite operations affected. G5 Extreme (Kp 9): aurora visible into the tropics, power grid collapse possible, satellite loss possible. The 1859 Carrington Event and the 1989 Quebec blackout are the reference G5 events.
How Long Storms Last
Most G1โG2 storms run 6โ12 hours. G3โG4 events often last 24โ36 hours with multiple substorm peaks. Historical G5 storms have lasted 2โ3 days as multiple CMEs pile in. The May 2024 Gannon Storm was the first G5 in more than 20 years.
Reading the Grid
On NOAA's dashboard, the G-scale bars fill up as Kp climbs. A steady G3 across multiple 3-hour windows is a substantially bigger event than a single-window G3 spike, even though both technically qualify.
Frequently Asked Questions
How rare is a G5 storm?
Roughly 4 per solar cycle historically, but the exact rate varies. Cycles 23 and 24 produced very few; Cycle 25 has been more active.
Which G-level threatens power grids?
G3 requires corrective action; G4 and G5 can cause voltage problems and, in extreme cases, transformer damage.