Space weather guide
The Kp Index Explained: How Far South Can You See the Aurora?
The Kp index is the number most aurora forecasts are built on. It runs from 0 to 9, and the higher it goes, the further from the poles the northern lights can be seen.
Updated 27 October 2026
What the Kp index measures
Kp measures how disturbed Earth's magnetic field is, based on magnetometers around the world. It is given for each three-hour block of the day on a scale from 0 (calm) to 9 (extreme storm). NOAA's Space Weather Prediction Center publishes the current value and a three-day forecast.
When a burst of solar wind or a coronal mass ejection (CME) hits Earth, the magnetic field is shaken up, Kp rises, and the auroral oval, the ring of aurora around each pole, grows and pushes towards the equator.
Which Kp you need where you live
These are rough guides for a dark, clear sky. At the lower end of each range the aurora will be low on the northern horizon (southern horizon in the southern hemisphere), and often easier to see in a phone photo than with the naked eye.
| Kp | NOAA storm level | Europe | North America |
|---|---|---|---|
| 3 | None | Iceland, northern Norway, Finland and Sweden | Alaska, northern Canada |
| 4 | None | Central Scandinavia, Shetland, northern Scotland | Southern Canada |
| 5 | G1 (minor) | Scotland, southern Norway and Sweden | Northern US states such as Washington, Minnesota and Maine |
| 6 | G2 (moderate) | Northern England, Ireland, Denmark, northern Germany and the Netherlands | New York, Wisconsin, Oregon |
| 7 | G3 (strong) | Most of the UK, Germany and Poland | Illinois, Pennsylvania, northern California |
| 8 | G4 (severe) | Central Europe, northern France and the Alps | Virginia, Kansas |
| 9 | G5 (extreme) | Southern Europe, as in May 2024 | Southern states, as in May 2024 |
Why a high Kp is not the whole story
- Kp is a three-hour average. The aurora comes in bursts, often lasting 15 to 30 minutes, so the best show can be over by the time a high Kp is published.
- The Bz matters. The direction of the magnetic field in the solar wind, called Bz, decides how much energy gets in. When Bz points south (negative) for a while, the aurora strengthens. Solar shows the live Bz from spacecraft 1.5 million km upstream of Earth, which gives about 30 to 60 minutes' warning.
- Darkness, cloud and the Moon decide whether you can see it at all. A bright Moon washes out a faint aurora.
That is why Solar's aurora score combines NOAA's OVATION aurora model with the cloud forecast, how dark your sky gets and the Moon, and turns it into a simple go, maybe or stay-in answer for your location.
Where the storms come from
Most strong storms are caused by CMEs, huge clouds of magnetised gas thrown out by the Sun, often alongside a solar flare. They take one to four days to reach Earth. NASA models each one and estimates when it will arrive and how strong the storm could be.
Smaller, regular storms come from fast solar wind streaming out of coronal holes, which can return every 27 days as the Sun rotates.