The Kp index is a 0–9 measure of how disturbed Earth's magnetic field is, updated every three hours. It sizes the auroral oval, not the aurora above you. What a given Kp means depends on your geomagnetic latitude: little inside the oval, a lot further from the pole, and inside the polar cap a rising Kp can mean less aurora.
What Kp actually measures
Kp is a planetary geomagnetic-activity index, an integer from 0 to 9, computed in eight three-hour UTC blocks a day. Julius Bartels introduced it in 1949. Two versions circulate: the definitive Kp, calculated by GFZ Potsdam from 13 magnetometer stations at geomagnetic latitude 44–60°, and NOAA SWPC's real-time "estimated planetary Kp," published ahead of the definitive value — the number most sites, including this one, show as "current Kp."
What it measures is disturbance of Earth's magnetic field, not aurora directly. A higher Kp means a larger auroral oval, stretched further from the pole — it is not a direct reading of how bright the sky is above any one point on the ground. Inside the oval itself, Kp tells you comparatively little, because the aurora is already a realistic possibility on most nights regardless of the index; the section on geomagnetic latitude below explains what that means in practice.
The 0–9 scale, in three-hour steps
Each Kp value applies to a fixed three-hour UTC block, so the number can change up to eight times a day as a new block starts. NOAA's reference relationship for the "quiet" oval's equatorward edge moves about 2° of geomagnetic latitude per Kp step:
| Kp | Oval's equatorward edge (geomagnetic latitude) |
|---|---|
| 0 | ~66° |
| 1 | ~64° |
| 2 | ~62° |
| 3 | ~60° |
| 4 | ~58° |
| 5 | ~56° |
| 6 | ~54° |
| 7 | ~52° |
| 8 | ~50° |
| 9 | ~48° |
This is an average relationship, not a hard line: NOAA also notes that a bright display can be seen up to roughly 1,000 km (600 miles) equatorward of the oval's actual edge, low on the horizon. Read the table as "where the odds shift," not as a wall the aurora stops at.
Kp and the NOAA G-scale
NOAA also reports geomagnetic storms on a separate G1–G5 scale, and the two map directly onto each other: G1 (Minor) is Kp 5, G2 (Moderate) is Kp 6, G3 (Strong) is Kp 7, G4 (Severe) is Kp 8, and G5 (Extreme) is Kp 9; anything below Kp 5 is below storm level. If a headline mentions a "G3 storm" or a forecast reads "G1," it is describing the same number this page is about, just under NOAA's storm-scale name instead of the raw index.
What Kp means at your geomagnetic latitude
This is the part most explainers skip, and it decides whether a Kp number is useful to you at all. Latitude here means geomagnetic latitude — measured from the magnetic pole, which can sit several degrees away from the geographic pole on a map — not the latitude a GPS shows. Every location page on this site carries its own geomagnetic latitude, computed once with the AACGM-v2 model the aurora literature relies on, rather than the simpler centred-dipole approximation, and recomputed as Earth's field drifts; how our forecast works covers the calculation.
Inside the auroral oval: Kp barely moves the needle
The "quiet" auroral oval sits in a thin band around 67–68° geomagnetic latitude, with its equatorward edge normally ranging roughly 65–75° depending on activity. For a location already inside that band, aurora is a realistic possibility on many nights independent of Kp — the UAF Geophysical Institute describes Kp inside the oval as a weak predictor, not a threshold to clear. Tromsø, well north of the Arctic Circle at roughly 69.6°N geographic latitude, sits under the oval geomagnetically too — see how our forecast works for the recalculated figure. Tromsø's own page treats even quiet nights as worth checking, rather than waiting for a specific Kp. Fairbanks, where the UAF Geophysical Institute watches the same sky, is in much the same position: cloud and darkness decide most nights there, not the index.
Sub-auroral and mid latitudes: Kp is doing the real work
Further from the pole, the calculus flips: the oval has to expand to reach you, which means you need Kp to climb, often into G-storm territory. Much of central and western Europe, and the northern-tier United States away from Alaska, fall in this zone. It is not a lost cause — the May 2024 "Gannon" storm reached Kp 9 (G5), the first Kp 9 since October 2003, and aurora was reported at unusually low latitudes that night. Cities such as Munich and London sit in this band: on an ordinary night the odds are low, and on a strong storm night they are genuinely in play. Each city's own page carries its specific minimum Kp, deliberately not repeated here — the API's threshold and a simple formula can disagree right at a band's edge, and a number printed in this guide would drift out of sync with the one that page actually shows.
Above the polar cap: a rising Kp can mean less aurora, not more
Above roughly 75–77° geomagnetic latitude, a location sits poleward of the oval, inside the polar cap. Because a rising Kp pushes the oval's edge further equatorward — further from the pole — a location already poleward of the oval sees that same expansion as the display moving away, not toward it. Discrete aurora is generally less frequent and weaker inside the polar cap than inside the oval itself. Longyearbyen, on Svalbard, sits in this situation; its own page is one of the clearest examples on the site of a place where "higher Kp" is not simply "better."
How far ahead a Kp forecast can be trusted
Not every number carries the same confidence, and the shortest-range one is not a Kp at all:
| Horizon | What drives it | How much to trust it |
|---|---|---|
| 27-day outlook | The Sun's ~27-day rotation bringing recurring active regions back around | A pattern to watch, not a forecast for one night |
| 3-day forecast | NOAA's Kp outlook feeding the OVATION aurora model | Useful for choosing which nights of a trip to prioritize |
| 30–90 minutes (a nowcast: a forecast of the next hour or so) | NOAA's OVATION model — a map of aurora probability, not a Kp value — limited by solar-wind travel time from L1 | The most reliable outlook this site shows — but only minutes of lead time |
Underneath all three sits the one input nobody can forecast: the interplanetary magnetic field's north–south component, Bz. Southward Bz is what actually lets solar-wind energy into the magnetosphere, and it is only measured about 1.5 million km upstream, at the L1 point — 15 to 60 minutes before it reaches Earth. A coronal mass ejection gives 1–3 days of notice that something is coming from the Sun, but not that Bz will cooperate once it arrives. That is why a 3-day Kp forecast is a reasonable planning tool and a poor source of certainty.
Where your own number lives
Every location page on this site prints a minimum Kp for that specific place — the Kp at which the oval's edge is expected to reach it, given its own geomagnetic latitude. That number already does the latitude work this page only describes in general terms; there is no need to re-derive it from the tables above. Planning around one specific destination instead of a general rule? Our guide to Kp in Iceland walks through a single country at that level of detail.
What this page doesn't tell you
Nothing here is a live value. Tonight's actual Kp, the multi-day outlook and the OVATION nowcast are on the home page and on every location page — this page explains what those numbers mean, not what they currently say. Kp is also only one of several inputs those pages weigh: cloud cover and darkness decide the rest of any given night, and how to read a forecast page walks through the whole picture. In every latitude band, Kp is a probability signal, not a guarantee — a higher number improves the odds; it does not promise a display. For everything else this page doesn't cover, browse all guides.