Geomagnetic Storms
A geomagnetic storm is a temporary disturbance of Earth's magnetic field, driven by bursts of solar wind from the Sun. NOAA ranks the biggest events on a five-step G1–G5 scale — the same activity that lights up the aurora and, at the top end, rattles power grids and satellites.
Open the live tracker →What a geomagnetic storm actually is
Earth sits inside a protective magnetic bubble, the magnetosphere. A geomagnetic storm happens when the solar wind — a stream of charged particles flowing out from the Sun at roughly 300–800 km/s — arrives carrying a magnetic field that points south (a negative Bz). When that southward field meets Earth's northward field, the two link up through a process called magnetic reconnection, and energy pours into the magnetosphere. That energy builds up an electric current ring around the planet and drives the auroral oval to expand toward the equator. The stronger and longer the southward tilt, the deeper the storm. If you want the switch behind it all, it lives in the Bz component of the solar wind.
What causes them: CMEs and coronal holes
Almost every significant storm traces back to one of two solar sources. Knowing which one is coming tells you a lot about how big and how sudden the storm will be.
Coronal mass ejections
A CME is a billion-tonne cloud of magnetized plasma flung off the Sun. Earth-directed CMEs take roughly 1–3 days to arrive and can slam the magnetosphere hard — these produce the strongest G3–G5 storms. Space weather →
Coronal-hole streams
Coronal holes are cooler, open regions of the Sun's corona that fire out fast, steady solar wind. They tend to drive milder, longer G1–G2 storms that often recur about every 27 days as the Sun rotates. Solar wind →
The southward switch
Neither source matters much unless its embedded field turns south. A fast wind with a northward Bz can pass almost quietly; a slower one with a strong southward Bz can storm. Bz & solar wind →
How we see it coming
Spacecraft parked at the L1 point, about 1.5 million km sunward, measure the wind 15–60 minutes before it reaches Earth — the final warning of storm strength. Aurora alerts →
The NOAA G1–G5 scale, level by level
NOAA's G-scale grades storm severity in five steps, each tied to a planetary KP value (KP runs 0–9) and to real-world effects. Higher levels are rarer but push the aurora dramatically farther south — and put more stress on technology.
G1 — Minor (KP 5)
Weak power-grid fluctuations and minor satellite-operation effects. Aurora visible across high latitudes — northern Scandinavia, Scotland, the northern US border states.
G2 — Moderate (KP 6)
High-latitude power systems may see voltage alarms; HF radio can fade near the poles. Aurora reaches down toward New York, the UK and central Europe.
G3 — Strong (KP 7)
Voltage corrections may be needed, satellites can accumulate surface charge, and GPS and HF radio degrade. Aurora seen as low as Illinois, Oregon and northern England.
G4 — Severe (KP 8)
Widespread voltage-control problems and possible protective-relay trips; drag increases on low satellites. Aurora can reach the mid-US and southern Europe.
G5 — Extreme (KP 9)
Grid systems can collapse, transformers may be damaged, HF radio can black out for a day or more, and satellite navigation is degraded. Aurora may be seen into the deep mid-latitudes.
Watching it live
See where activity sits on the scale right now, and where the glow is centred over the pole. Aurora map →
Storms that made history
A few events show what the top of the scale really means. The Carrington Event of 1859 — the most intense geomagnetic storm on record — set telegraph lines sparking and pushed aurora as far as the Caribbean. In March 1989, a severe storm collapsed the Hydro-Québec power grid and left about six million people without electricity for roughly nine hours. The Halloween storms of October 2003 reached G5, forced flights off polar routes and disrupted satellites. More recently, the May 2024 “Gannon” storm was the first G5 since 2003 and brought the aurora to unusually low latitudes worldwide. None of these harmed people on the ground directly — the danger of an extreme storm is to infrastructure, not to bodies.
Geomagnetic storm FAQ
How is the G-scale different from the KP index?
They describe the same activity from two angles. The KP index is the raw 0–9 measure of global geomagnetic disturbance; the G-scale starts where storms begin and translates severity into expected effects. They map directly: G1 = KP 5, G2 = KP 6, G3 = KP 7, G4 = KP 8 and G5 = KP 9. Learn about the KP index →
How much warning do we get before a storm hits?
It depends on the source. When a CME erupts, observatories usually see it within hours and can estimate arrival 1–3 days out, but the crucial detail — whether its field is southward — is only confirmed 15–60 minutes ahead by spacecraft at the L1 point. Coronal-hole storms are more predictable, often recurring on the Sun's ~27-day rotation. More on space weather →
Are geomagnetic storms dangerous to people on the ground?
No — the atmosphere and magnetosphere shield us, so even an extreme storm poses no direct health risk at the surface. The real impacts are technological: power-grid stress, transformer damage, satellite drag and charging, GPS errors and HF radio blackouts. The 1859 and 1989 storms are the classic warnings of what a severe event can do to infrastructure.
Watch geomagnetic storms unfold live
Real-time KP index, Bz, solar wind speed and a single 0–9 Aurora Power score — so you can see a storm building and know when the aurora is on the move.
Open the live tracker →