Geomagnetic Storm Scale
Explained (G1–G5)
When the sun sends a burst of charged particles toward Earth, NOAA rates the resulting disturbance on a G1–G5 scale. Here's what causes a geomagnetic storm, and what each level actually does.
What Causes a Geomagnetic Storm
A geomagnetic storm happens when a burst of solar wind — usually from a coronal mass ejection (CME) or a high-speed stream from a coronal hole — slams into Earth's magnetic field. The impact compresses and disturbs the magnetosphere, driving electrical currents through the upper atmosphere and near-Earth space.
Those currents are what produce the aurora, and at higher intensities, what can disrupt satellites, radio signals, GPS accuracy, and power grids.
The NOAA G-Scale
NOAA's Space Weather Prediction Center rates geomagnetic storms G1 (minor) through G5 (extreme), based on the Kp index — a 0–9 measurement of geomagnetic disturbance taken every 3 hours.
G1 — Minor (Kp 5)
Weak power grid fluctuations. Minor impact on satellite operations. Aurora visible from the northern-most US states. Happens roughly 900 days per 11-year solar cycle — common.
G2 — Moderate (Kp 6)
High-latitude power systems may see voltage alarms. Some spacecraft experience minor drag increases. Aurora reaches the northern half of the US.
G3 — Strong (Kp 7)
Voltage corrections may be required on power systems. Satellite navigation (GPS) and low-frequency radio can experience intermittent problems. Aurora visible in mid-latitude states.
G4 — Severe (Kp 8)
Widespread voltage control problems possible; some grid systems may experience protective relay trips. HF radio propagation is sporadic. Aurora visible as far south as Alabama and Northern California. Happens a handful of times per solar cycle.
G5 — Extreme (Kp 9)
Widespread voltage control problems and possible grid collapse or blackouts. Transformers can be damaged. HF radio may be unusable for hours. Aurora visible from Florida and Texas. Extremely rare — the May 2024 storm was the first G5 since 2003.
What a G-Storm Actually Means for You
Most people: nothing noticeable
Below G3, geomagnetic storms don't affect daily life for most people. The main visible effect is aurora, if you're far enough north and skies are clear.
G3+: GPS and HF radio drift
Precision GPS (surveying, agriculture) can show small errors. Amateur and aviation HF radio operators may notice degraded or dropped signals, especially over polar routes.
G4–G5: grid operators take action
Utility companies actively monitor space weather forecasts and can adjust grid operations ahead of a severe storm to reduce risk of damage or blackouts — this is the primary reason NOAA forecasts these events days in advance.
Notable Storms in History
1859 Carrington Event
The most intense geomagnetic storm on record, from a CME so powerful it induced currents strong enough to shock telegraph operators and set some telegraph paper on fire. A storm of this scale today would pose a serious risk to modern satellites and power grids.
1989 Quebec Blackout
A severe geomagnetic storm induced currents in Hydro-Québec's power grid that tripped protective relays, causing a 9-hour blackout across the province affecting millions of people — a real-world demonstration of the grid-impact risk described in the G4–G5 tiers above.
May 2024 Gannon Storm
The first G5 storm since 2003, producing aurora visible across nearly the entire continental US, including Florida and Texas. Named after the space physicist Jennifer Gannon, who died shortly before the event.
Track Space Weather in StormCast
StormCast's Space Weather dashboard shows the live Kp index, current G-scale storm level, solar wind data, and CME arrival forecasts — plus push notifications when a geomagnetic storm is strong enough to bring the aurora to your latitude.
