Geomagnetic Storm
Definition
A major disturbance of Earth's magnetic field caused by solar wind structures, rated G1 (minor) to G5 (extreme).
In plain language
When a strong gust of solar wind โ usually a CME โ hits Earth's magnetic bubble, the whole field shakes for hours or days. During these storms the auroral ovals expand and brighten dramatically.
The science
Storms occur when sustained southward IMF drives efficient dayside reconnection, injecting energy that intensifies the ring current and expands the auroral ovals equatorward. NOAA's G-scale maps Kp 5โ9 to G1โG5.
Why it matters for aurora viewing
Storm level tells you how far from the poles the aurora can reach: G1 touches the northern US; G4โG5 can light up much of the continent.
See this in action on tonight's data.