SKY · FIELD GUIDE
The Kp Index, Explained
Every aurora forecast leads with a single number — 'Kp 5 tonight.' Here's what that number is, where it comes from, and the Kp you actually need to see the lights from where you are.
If you only learn one number for aurora-watching, make it Kp. It's the shorthand the whole field uses, and once you know what it means you can read any forecast at a glance.
What it actually measures
Kp is a 0–9 index of how disturbed Earth's magnetic field is, calculated over three-hour windows. It's built from a network of ground magnetometers spread around the planet that detect how much the local magnetic field is wobbling. Average those wobbles, scale them, and you get a single planetary number — the "p" in Kp stands for planetarische (planetary).
A calm, undisturbed field scores low. A field being buffeted by a burst of solar wind — a geomagnetic storm — scores high. Crucially, a higher Kp means the auroral oval grows and slides toward the equator, bringing the lights within reach of more people.
What each level means
| Kp | Field state | Roughly visible from |
|---|---|---|
| 0–2 | Quiet | The far Arctic / Antarctic only |
| 3–4 | Unsettled to active | Scandinavia, Iceland, Alaska, northern Canada |
| 5 (G1) | Minor storm | Far-northern US states, much of the UK |
| 6 (G2) | Moderate storm | Northern US, southern England, Germany |
| 7 (G3) | Strong storm | Central US and central Europe |
| 8 (G4) | Severe storm | Southern US, southern Europe |
| 9 (G5) | Extreme storm | Far into the mid-latitudes — a rare, headline event |
These are typical boundaries, not guarantees: local conditions, the time of night, and how dark your sky is all matter.
Kp and the G-scale
You'll often see a G-number next to the Kp. That's NOAA's storm-severity scale, and it maps directly onto the top of the Kp range:
- G1 ≈ Kp 5 (minor)
- G2 ≈ Kp 6 (moderate)
- G3 ≈ Kp 7 (strong)
- G4 ≈ Kp 8 (severe)
- G5 ≈ Kp 9 (extreme)
Below Kp 5 there's no G-rating — the field is just "active" or "quiet."
The number that really matters: your threshold
Here's the part most forecasts leave out. The headline Kp tells you how active the field is — but whether you can see anything depends on your geomagnetic latitude. Two numbers, side by side:
- Current Kp — what the Sun is doing right now.
- Your threshold — the Kp at which the oval reaches your location.
If the current Kp meets or beats your threshold, the sky is clear, and you can get away from light pollution, it's worth going out. That side-by-side comparison is exactly what a location-aware aurora tracker does — it does the latitude maths for you and turns the raw number into a plain yes-or-no for tonight.
Frequently asked questions
What is the Kp index?
The Kp index is a 0–9 scale describing how disturbed Earth's magnetic field is over a three-hour window, averaged from magnetometer readings around the world. Low numbers mean a calm field; high numbers mean a geomagnetic storm — and a better chance of aurora at lower latitudes.
What Kp do I need to see the aurora?
It depends on your geomagnetic latitude. Far-north places like Tromsø or Fairbanks see aurora at almost any active level. Mid-latitude cities like Edinburgh or Seattle typically need Kp 5–6; places further south may need Kp 7 or higher. A location-aware tracker compares the live Kp to your threshold for you.
How is Kp related to the G-scale?
NOAA's G-scale rates geomagnetic storm strength: G1 (minor) is about Kp 5, G2 Kp 6, G3 Kp 7, G4 Kp 8, and G5 (extreme) Kp 9. Below Kp 5 there's no storm rating — the field is merely 'active' or 'quiet.'
How often does the Kp index update?
NOAA's Space Weather Prediction Center publishes an estimated Kp roughly every few minutes from real-time data, with the formal index assigned in three-hour blocks. Live trackers read the most recent estimate, so the number can change through the night.
SEE IT LIVE
Everything in this guide is on the live sky map.