What is the Kp index?
The short version: Kp is a score from 0 to 9 for how stirred up the Earth’s magnetic field is. The higher the Kp, the further south the aurora oval pushes — and the better your chances of seeing the northern lights. It is the headline number in every aurora forecast, including ours.
Where the number comes from
Kp is measured by a network of magnetometer stations around the world and published by NOAA’s Space Weather Prediction Center as one average for the whole planet, updated every 3 hours. That 3-hour grain matters: a short, bright burst can live entirely inside one block, which is one reason a forecast can be right about the night and still miss the best twenty minutes.
The scale, in plain words
| Kp 0–1 | Quiet. The oval sits far north; realistically a Fairbanks-and-north show. |
|---|---|
| Kp 2 | Unsettled. Fairbanks gets a proper show; Anchorage watches the northern horizon. |
| Kp 3 | Active. Anchorage’s horizon show; overhead further north. |
| Kp 4 | Active. A good Anchorage night; the band reaches the Southeast on the horizon. |
| Kp 5 | A geomagnetic storm (G1, minor). Overhead in Anchorage; Ketchikan gets its show. |
| Kp 6–7 | G2–G3 storms. Strong, widespread displays; most of Alaska is under the oval. |
| Kp 8–9 | G4–G5, rare and extreme. The oval can push so far south that the show moves south of northern towns — and the Lower 48 gets a turn. |
How much does your town need?
It depends on magnetic latitude — how close your town sits to the magnetic pole, which is not the same as geographic latitude. The oval’s southern edge reaches about magnetic latitude 69 − 3×Kp at local midnight, so: Fairbanks needs about Kp 2, Anchorage about Kp 3, Ketchikan about Kp 4 for the band to reach the horizon, and roughly 2 more for a strong show directly overhead. Every town page on this site states the Kp that town needs — the verdict already does this comparison for you.
Why Kp alone doesn’t decide your night
Three honest caveats. First, Kp is a planetary average — Alaska can outperform or underperform it on any given night. Second, brightness also depends on the solar wind’s speed, density, and magnetic direction (a southward “Bz” opens the tap; Kp folds all of that into one blunt number). Third, Kp says nothing about your sky: solid cloud, a short summer night, or a bright moon can cancel a Kp 6. That is why this site’s verdict combines Kp with cloud cover, your town’s darkness window, and moonlight before it gives you a word instead of a number.
How to use it
Find the Kp your town needs, then compare it with tonight’s forecast peak: at or above your number, it is worth going out and looking north; two above it, expect the show overhead. And if the sky is clear and you are already up, look anyway — Kp is odds, not a promise, in both directions.