Kp Index Explained

Kp is the number most aurora forecasts lead with. It is useful, but it is a summary of the past few hours, not a promise for the next few minutes.

What Kp measures

Magnetic observatories around the world record small swings in Earth's magnetic field. Each station turns its swings over a 3-hour block into a local K value from 0 to 9. The planetary Kp combines several of these stations into one global number.

The day is split into eight 3-hour blocks in Universal Time (UTC): 00-03, 03-06, and so on. Each block gets one Kp value.

The scale is quasi-logarithmic. A jump from Kp 7 to Kp 8 means a much bigger disturbance than a jump from 2 to 3.

Thirds: 5-, 5 and 5+

Kp is reported in thirds. Between 5 and 6 there are steps: 5- (a bit under 5), 5 (sometimes written 5o), 5+ (a bit over 5), then 6-.

Some forecasts show decimals instead, such as 4.67 or 5.33. These mean the same thing: 4.67 is 5-, and 5.33 is 5+.

Kp and the NOAA storm scale

NOAA's Space Weather Prediction Center uses a G scale for geomagnetic storms, tied directly to Kp.

For example, the storm of May 10-11, 2024 reached Kp 9 (G5). The storm of October 10-11, 2024 reached G4.

Estimated vs observed Kp

There are two kinds of Kp numbers you may come across.

• Estimated Kp is calculated in near real time by NOAA from a set of ground magnetometers. It updates often during the night and is what live displays usually show.
• Observed or definitive Kp is the official value. It is published by GFZ, the German Research Centre for Geosciences, after the data has been checked. It can differ slightly from the estimate.

Forecast Kp is a third kind: a prediction for each 3-hour block in the coming days. Our 3-day forecast shows these values, and the Kp index page shows the live estimate. See our method for the data sources.

Why Kp is only a rough guide

Kp is an average over 3 hours and over the whole planet. The aurora can change in minutes and varies by region. So a Kp 4 night can bring a sharp burst of activity, and a forecast Kp 6 night can stay quiet.

Two real-time solar wind values help fill the gap:

• Bz, the north-south direction of the solar wind's magnetic field. When Bz points south (negative) for a while, energy flows into Earth's magnetic field and aurora tends to build.
• Solar wind speed and density. Faster, denser wind pushes harder on the magnetosphere. Speeds well above normal often go with stronger displays.

A steady negative Bz with fast solar wind is often a better short-term sign than the Kp number alone.

Using Kp for your location

The Kp you need depends on your geomagnetic latitude. Northern Alaska may see aurora at Kp 1 or 2, while a central US state may need Kp 7 or more. Our state pages and how far south guide explain the typical values.

Questions people ask