The G-scale and Kp
The G-scale was introduced by NOAA to make geomagnetic storm severity accessible to non-specialists. Each level maps to a Kp range: G1 corresponds to Kp 5, G2 to Kp 6, G3 to Kp 7, G4 to Kp 8, and G5 to Kp 9.
NOAA publishes storm watches (likely in 1-3 days), warnings (expected within 24 hours), and alerts (in progress now). Aurora Tonight uses the same underlying Kp data to power its live forecast and push notifications.
G-scale at a glance
What happens at each level
Geomagnetic storms affect more than aurora. NOAA monitors infrastructure impacts at each level. Here is what each G category means beyond aurora visibility.
G1 - Minor
Weak power grid fluctuations. Minor satellite drag. Aurora visible from Norway, Iceland, and Alaska. The most common storm level - occurs around 1,700 times per solar cycle.
G2 - Moderate
High-latitude power grids may experience voltage fluctuations. Satellite drag becomes noticeable. HF radio propagation degrades at higher latitudes. Aurora visible from Scotland and southern Scandinavia.
G3 - Strong
Satellite surface charging may occur. Intermittent navigation problems on radio. Voltage corrections may be required on power systems. Aurora visible from northern England and the northern US states.
G4 - Severe
Wide-area power grid problems are possible. Satellite tracking becomes difficult. Induced pipeline currents measurable. Aurora visible from southern England, France, and Germany.
G5 - Extreme
Widespread voltage control problems. Possible grid collapses in some areas. Satellite drag extreme - operators may lose track of objects. Radio blackouts widespread across multiple frequency bands. Aurora visible from 40°N including Spain and the southern US.
How often does each level occur?
During solar minimum, G3+ events are rare - sometimes none occur for months. During solar maximum, which covers 2025-2026 for the current cycle, G2-G3 events occur several times per month. G4 events happen a few times per year at maximum.
G5 events are exceptional: fewer than 4 occur per 11-year solar cycle. The May 2024 storm reached G5 and produced aurora visible across the continental US, Europe, and New Zealand. It was the strongest event since the Halloween storms of October 2003. Most aurora watchers will never see another G5 in this cycle.
