What is the KP index?

Where the number comes from

The "K" in KP stands for "Kennziffer", a German word that roughly translates to "characteristic figure". The index was invented in the late 1940s by a German geophysicist named Julius Bartels, who needed a quick way to summarise readings from a global network of magnetometers.

Today the value is computed every three hours by combining data from 13 ground stations spread across the planet. NOAA in the United States and a partnership of European institutes publish the result. Apps and websites pull from these public feeds.

One important thing: KP is the planetary number. There is also a local K index, measured at one specific magnetometer, but most public services quote KP because it is a single global score you can act on.

What each level means

KP	Geomagnetic state	Aurora visible from
0-2	Quiet	Far north only: Tromso, Fairbanks, Murmansk
3	Unsettled	Northern Iceland, northern Norway, central Alaska
4	Active	Reykjavik, Anchorage, Helsinki, Yellowknife
5	Minor storm	Stockholm, Oslo, Edinburgh, Calgary, parts of US/Canada border
6	Moderate storm	Northern UK, Berlin, Warsaw, US states bordering Canada
7	Strong storm	London, Amsterdam, Berlin, northern US states
8-9	Severe / extreme storm	Mid-latitude Europe, central US, occasionally further south

These are likelihoods, not guarantees. A KP of 5 is necessary for you to see aurora from northern UK, but it is not sufficient. Cloud cover, light pollution, time of night, and where the aurora oval happens to be lobing all matter.

How fast it changes

KP is published in three-hour windows, so the headline number you see is always slightly behind reality. For real-time use, look at the estimated KP that the same feeds publish every hour, or at the aurora oval map, which updates every few minutes.

Storms also come in waves. The big geomagnetic storm of May 2024 hit KP 9 for several hours, dropped back to KP 6, then climbed again the following night. A high KP one evening does not mean the next evening will be quiet, especially when an active coronal mass ejection is still arriving.

A common misunderstanding

People sometimes ask: "If KP is 5, will I definitely see green?" The answer is no, and not because the number is wrong. KP is a measure of magnetic disturbance, not of how bright the light is. You can have a KP 5 night where the aurora is faint but visible all the way down to Edinburgh, and another KP 5 night where it stays low on the horizon and only shows up in long-exposure photos. The camera is much more sensitive than your eyes.

If you want to know whether tonight is worth driving out for, look at KP and Bz (the southward orientation of the solar wind's magnetic field). A KP of 4 with a strongly southward Bz often beats a KP 6 with a northward Bz.