What the Kp index actually measures
The planetary Kp index is a three-hourly measure of how disturbed Earth's magnetic field is, derived from a global network of ground-based magnetometers. Each station computes a local K-index from how far its horizontal magnetic field reading has swung from its usual quiet-day baseline, those local readings are standardised so that a station near the pole doesn't simply always outscore one near the equator, and the results are averaged into a single number from 0 to 9. It is not a measurement of the aurora itself -- it is a measurement of geomagnetic disturbance, which happens to correlate closely with auroral activity because both are driven by the same underlying process: the solar wind interacting with Earth's magnetosphere. For a deeper walkthrough of the history and mechanics, seewhat the Kp index is.
Two versions of Kp circulate in practice. The official, definitive Kp is only finalised after the fact from processed magnetometer data, on a three-hour cadence. A near-real-timeestimated Kp, refreshed roughly every minute, is what this gauge (and most live aurora sites) actually displays, since waiting three hours for a "confirmed" number defeats the purpose of a live dashboard. The two occasionally diverge by half a step or so mid-window; that is normal and not a sign either value is wrong.
From Kp to the G-scale
Kp values of 5 and above correspond to NOAA's G-scale for geomagnetic storms, which the agency uses to communicate space-weather severity to the public and to industries such as satellite operators and power utilities:
| Kp | G-scale | Description |
|---|---|---|
| 0-4 | G0 (Quiet) | No storm. Aurora, if any, stays confined to high latitudes. |
| 5 | G1 Minor | Weak storm. Aurora may become visible from the northern-tier US and similar latitudes. |
| 6 | G2 Moderate | Aurora can reach further, into parts of the northern US, UK and similar bands. |
| 7 | G3 Strong | Aurora regularly reported at mid-latitudes; a genuinely good night for most auroral-zone destinations. |
| 8 | G4 Severe | Widely reported aurora well into mid-latitudes; a rare, notable event. |
| 9 | G5 Extreme | Historic storm level; aurora has been reported as far as the sub-tropics on past occasions. |
This site's own city pages, and the gauge above, use a closely related but continuous Kp-to-geomagnetic-latitude relationship (roughly 66.5 degrees at Kp 0, dropping about two degrees per step) rather than the coarser integer G-scale bands, since it produces a more precise threshold per location. Seethe NOAA G-scale explained for the full detail on what each level means in practice.
What this means for your latitude
Because the aurora forms an oval roughly centred on the geomagnetic pole rather than the geographic one, "how high a Kp you need" depends on your geomagnetic latitude, not just how far north or south you live on a map. Reykjavik and Anchorage sit at similar geographic latitudes to some interior Canadian cities but have noticeably different geomagnetic latitudes, and therefore different Kp thresholds. Every city page on this site computes and displays this threshold specifically, and the tonight page combines it with the current forecast, darkness, and moon phase into a single, fully explained verdict for your exact location -- start there if you want a direct answer for tonight rather than a general-purpose number.
How to actually use this forecast
Treat the current estimated Kp as the best available snapshot, and the 48-hour forecast strip above as a rough outlook rather than a guarantee -- geomagnetic forecasting genuinely gets more reliable the closer to real time it is, because the physical driver (the solar wind) is only measured with any useful lead time once it is about 30 to 90 minutes from Earth. A forecast Kp of 5 published a day in advance is a reasonable heads-up to keep an eye on conditions; the estimated Kp updating minute to minute is the number to actually act on when deciding whether to go outside tonight.
Frequently asked questions
What counts as a "good" Kp for the aurora?
It depends entirely on your latitude. Kp 3-4 is often enough for high-latitude locations such as Tromso or Fairbanks, while a mid-latitude location such as London or Chicago typically needs Kp 6-7 or higher. Use /tonight/ to get the specific threshold for your own location.
Why does the gauge sometimes disagree with the number I see elsewhere?
Different sites use either the official 3-hour observed Kp or the 1-minute estimated Kp, which can differ slightly, especially mid-window. This page uses the estimated value when it is available, since it is the more current of the two, and falls back to the last official observation otherwise.
Does the forecast strip show a full 3-day outlook?
It shows NOAA's near-term Kp forecast in 3-hour steps. NOAA publishes roughly two days ahead, so the strip usually covers about 48 hours. Longer-range outlooks exist but carry much lower confidence -- geomagnetic storms are driven by solar events that are themselves only reliably observed 1-3 days in advance at best.
Is a G-scale storm the same thing as "the aurora is strong"?
Related but not identical. The G-scale describes geomagnetic storm intensity (Kp 5 and above), which correlates strongly with a brighter, wider aurora, but the two aren't a perfect one-to-one match -- NOAA's OVATION model, shown on the home page map, is a more direct read on the aurora oval itself.