How Geomagnetic Storms Are Classified (G1–G5)

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 G1–G2 storms run 6–12 hours. G3–G4 events often last 24–36 hours with multiple substorm peaks. Historical G5 storms have lasted 2–3 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.

Sources

NOAA Space Weather Scales (G, S and R) — NOAA SWPC — Official G1–G5, S1–S5 and R1–R5 definitions and average frequency per solar cycle.
Geomagnetic Storms — NOAA SWPC — How CMEs and solar wind drive storms and the Kp-based G-scale.
Planetary K-index — NOAA SWPC — Kp is an integer 0–9; 5 or more indicates a geomagnetic storm.

Canonical Reading

Learn more about Geomagnetic Storms