How aurora forecasts work
Aurora forecasts combine measurements from the Sun, from spacecraft in the solar wind and from magnetometers on the ground. Each tells you something different, over a different time horizon.
The Kp index
Kp measures global geomagnetic disturbance in three-hour blocks, on a scale from 0 to 9 in thirds (2−, 2o, 2+ …). It is derived from magnetometers at mid-latitude observatories and published by the GFZ Helmholtz Centre in Potsdam; NOAA publishes an estimated real-time version and a three-day forecast.
The higher the Kp, the further the auroral oval expands away from the pole. But Kp is a global average over three hours: a short burst can produce a great display on a 'Kp 3' night, and a high-latitude place can see aurora at Kp 0.
NOAA's G-scale
Geomagnetic storms are rated G1 (Kp 5) to G5 (Kp 9). G1–G2 storms bring the aurora to Scotland, southern Scandinavia and the northern US; G4–G5 storms can push it to mid-latitudes, as in May and October 2024.
Solar wind and Bz: the next hour
Spacecraft about 1.5 million kilometres towards the Sun measure the solar wind before it reaches Earth, typically 30 to 60 minutes later. Speed, density and especially Bz — the north–south direction of the interplanetary magnetic field — decide how much energy enters Earth's magnetic field.
A southward (negative) Bz couples the solar wind to the magnetosphere. Several nanotesla south for half an hour or more is the best short-term sign of a coming display.
The OVATION model
NOAA's OVATION model turns the measured solar wind into a map of the probability of visible aurora for the next 30 minutes. We show it on every place page and use it for the current hour of the score.
Days and weeks ahead
Coronal mass ejections take one to three days to reach Earth, so the three-day forecast depends on eruptions already seen. Fast streams from coronal holes often return every 27 days with the Sun's rotation, which gives a rough outlook a few weeks ahead. Anything further out is only a seasonal guide.
Why forecasts still miss
The arrival time of a CME is uncertain by many hours, and its magnetic orientation is only known when it passes the spacecraft. That is why a forecast 'storm' can fizzle, and why a quiet forecast night sometimes surprises.
Last reviewed 15 October 2026