KP Index
The KP index is the world's standard 0–9 gauge of global geomagnetic activity. It tells you, in a single number, how disturbed Earth's magnetic field is right now — and roughly how far from the poles the aurora is likely to reach.
See the live KP index →What the KP index actually measures
“KP” stands for planetarische Kennziffer — German for “planetary index,” a name that survives from its 1949 origin. It is a quasi-logarithmic scale from 0 to 9 that summarises how much Earth's magnetic field has been rattled by the solar wind over a fixed window. A KP of 0–1 means the field is calm; 4 is unsettled; 5 and above marks an official geomagnetic storm. Because the scale is quasi-logarithmic, each step up represents a disproportionately larger disturbance — the jump from KP 7 to KP 8 is far bigger than from KP 2 to KP 3.
Crucially, KP is derived from ground-based magnetometers, not from a satellite pointed at the sky. A global network of 13 geomagnetic observatories at sub-auroral latitudes each measure how far their local field wandered from a quiet-day baseline over a three-hour block. Those local “K” values are standardised and averaged into the single planetary number you see quoted everywhere. Reported values step in thirds — you will see figures like 4-, 5o and 6+ rather than plain integers.
The three-hour cadence — and why KP lags
The definitive KP index is published in eight three-hour blocks per day (00–03 UT, 03–06 UT, and so on). That cadence is its greatest strength and its biggest weakness. It is a robust, globally averaged record that has been kept consistently since 1932, which makes it superb for comparing storms across decades. But it is slow and backward-looking: a value is only final once its full three-hour window has closed, so KP can lag a real geomagnetic event by up to three hours.
To bridge that gap, forecasters use faster products. The estimated Kp is a provisional value updated roughly every minute from a subset of reporting stations — useful in the moment, but subject to revision. The Wing Kp model goes a step further, using solar-wind measurements from a spacecraft at the L1 point (about 1.5 million km sunward of Earth) to predict Kp one hour and four hours ahead. For deciding whether to head out tonight, the real-time Bz and solar-wind numbers are more actionable than a three-hour-old KP — they respond in minutes, not hours.
What KP do you need at your latitude?
There is no single “aurora KP” — the number you need depends entirely on how close you live to the auroral oval. The higher your geomagnetic latitude, the lower the KP required. The table below is a rough guide to the minimum KP at which aurora may become visible overhead on a clear, dark night; catching it low on the horizon can happen a step or two earlier.
| KP | Aurora roughly overhead near… | Activity level |
|---|---|---|
| 2–3 | Tromsø, Fairbanks, Yellowknife, northern Iceland | Quiet to unsettled |
| 4 | Reykjavík, Anchorage, Trondheim, far northern Scotland | Active |
| 5 | Oslo, Stockholm, Edinburgh, Calgary (G1 storm) | Minor storm |
| 6–7 | Southern Scandinavia, northern England, US–Canada border states (G2–G3) | Moderate–strong storm |
| 8–9 | Central Europe, mid US states, occasionally lower (G4–G5) | Severe–extreme storm |
These are typical thresholds, not guarantees. A storm with a strongly southward Bz holding for hours can push the lights further south than the headline KP suggests, while a brief KP spike may fade before you get outside. Use the table as a starting point, then check whether it's worth going out with our northern lights tonight read.
How KP maps to the NOAA G-scale
When KP reaches storm level, NOAA translates it into the five-step G-scale (G1–G5) used for public alerts and impact warnings. The mapping is direct: the storm categories are simply the top of the KP scale relabelled by severity, with each level carrying expected effects on power grids, satellites, GPS and radio.
KP 5 → G1 Minor
Weak power-grid fluctuations; aurora at high latitudes. The most common storm level.
KP 6 → G2 Moderate
Aurora reaches mid-latitudes; possible transformer and satellite-drag effects.
KP 7–8 → G3–G4
Strong to severe: widespread aurora, GPS and HF-radio degradation, grid voltage issues.
KP 9 → G5 Extreme
Rare, spectacular displays far from the poles — and the biggest risk to infrastructure. Geomagnetic storms →
The limits of a single global number
KP is powerful precisely because it compresses the whole planet into one figure — but that is also its blind spot. It is a global average, so it cannot tell you that the auroral oval happens to be brightening over your longitude while it is quiet elsewhere. It is also coarse and slow: three-hour resolution smooths over the sharp, minutes-long substorm surges that often produce the most dramatic overhead displays.
That is why experienced aurora watchers treat KP as context rather than a trigger. The faster switches — a sharply southward Bz, a jump in solar-wind speed and density arriving from the Sun, and a live view of the oval — react in real time. KP tells you the storm's overall scale; the L1 spacecraft data tells you what the next half hour holds. Read together, they give a far better picture than any one number alone. For the fuller physics, see our space weather overview.
KP index FAQ
Is a higher KP always better for seeing the aurora?
Not necessarily for you specifically. A higher KP means the auroral oval has expanded toward the equator, which helps lower-latitude viewers. But if you already live under the oval — say in northern Norway or Alaska — a very high KP can push the brightest band south of you, leaving fainter aurora overhead. The ideal KP is the one that places the oval directly over your location, which depends on your geomagnetic latitude.
Why is the KP I see quoted sometimes revised later?
Because there are two different products. The fast “estimated Kp” is a provisional real-time value based on a limited set of reporting stations and is updated every minute. The definitive KP is calculated after each three-hour window closes using the full 13-station network, so it can differ from the earlier estimate. If a number changes an hour later, you were likely looking at the estimate first and the final value second.
Can the aurora appear even when KP is low?
Yes. KP is a three-hour average, so a short, sharp substorm can light up the sky for ten or twenty minutes without moving the block value much. And if Bz turns strongly and steadily southward, energy pours into the magnetosphere in a way a single number captures only after the fact. This is exactly why watching Bz and solar wind live catches displays that a KP number alone would miss.
Watch the KP index update live
See the real-time KP index alongside Bz, solar-wind speed and density and a single 0–9 Aurora Power score — refreshed every minute from satellite data.
Open the live tracker →