The Kp index explained: what the number means for the aurora

The Kp index is a number from 0 to 9 that describes how disturbed Earth's magnetic field is, measured over three hours at a time. The higher the Kp, the further south the auroral oval reaches and, as a rule, the brighter the aurora. In Northern Norway you can see the northern lights at Kp 1–2, while southern Norway usually needs Kp 5 or more. But Kp knows nothing about clouds or darkness, and it is a coarse global average rather than a local measurement.

What Kp actually measures

When the solar wind shakes Earth's magnetic field, magnetometers on the ground record the field swinging more than usual. For every three-hour period each station gets a K value from 0 to 9, based on the largest swing in the horizontal part of the field. The letter K comes from the German word "Kennziffer" (roughly "characteristic number"), and the system was introduced by the German geophysicist Julius Bartels in 1938.

The planetary index, Kp, is the average of standardised K values from 13 observatories between 44 and 60 degrees geomagnetic latitude, eleven in the northern and two in the southern hemisphere. The standardisation removes differences caused by season and time of day, so that the stations can be compared. The official Kp series is calculated by the German research centre GFZ in Potsdam and goes back to 1932.

Two details are worth knowing:

• The scale is quasi-logarithmic. The step from Kp 8 to 9 is far larger in nanotesla than the step from 0 to 1. A jump from Kp 3 to 5 is more than "a bit more".
• Kp comes in thirds. The official notation has 28 steps: 0o, 0+, 1-, 1o, 1+, 2- and so on up to 9o. NOAA forecasts show the same steps as decimals: 4.33 means 4+, and 4.67 means 5-.

Official Kp and "estimated" Kp

The definitive Kp from GFZ is published after the fact. For aurora hunting you will almost always use NOAA's estimated Kp, which the US Space Weather Prediction Center (SWPC) calculates continuously from a smaller network of magnetometers in places such as Alaska, Canada, the UK, Germany and Australia. Their chart updates every minute, but each bar still covers a full three-hour window.

When you see a Kp forecast for the coming days, it is a forecast of this estimated number. How to read the forecasts themselves is covered in our guide to reading an aurora forecast.

Kp and the G1–G5 storm scale

NOAA translates high Kp values into a storm scale that turns up in news reports and alerts:

The last column is NOAA's own average for a solar cycle of roughly eleven years. It shows how rare the truly big storms are, and why a G4 or G5 night makes headlines far south in Europe. The background is in our article on geomagnetic storms.

Which Kp do you need where you are?

The aurora forms in a ring around the geomagnetic pole, the auroral oval. According to Store norske leksikon (the Norwegian national encyclopedia), the night side of the oval normally sits at about 67 degrees magnetic latitude, which in our part of the world means it follows the coast of Troms and Finnmark in the evening and at night. That is why Tromsø, Alta and Lofoten can have aurora almost straight overhead when Kp is low.

NOAA gives a rough rule for how the oval moves: at Kp 0 its southern edge lies around 66 degrees magnetic latitude, and it shifts about 2 degrees south for every Kp step, reaching around 48 degrees at Kp 9. The rule applies to magnetic, not geographic, latitude. Because the geomagnetic north pole sits over the Canadian Arctic, magnetic latitude in Norway is somewhat lower than the geographic latitude on your map.

On top of that, the aurora can be seen from a distance: NOAA notes that a bright display can be visible from as far as about 1000 kilometres away in good conditions, though then low in the sky.

Based on this, Nordlys i natt? uses these rules of thumb:

Treat the table as a starting point, not as hard physics. Different sources draw the lines slightly differently, and the aurora does not respect fixed boundaries. In the south and east of Norway, aurora at the threshold value will usually sit low in the north, and a camera will show it more clearly than your eyes. More on this in our articles on the aurora in southern Norway and the aurora in Tromsø.

The limits of Kp

Kp is handy, but it is easy to lean on it too hard:

• Three hours is a long time. An auroral outburst (a substorm) can flare up and fade within minutes. Kp smooths such peaks out, and a "quiet" three-hour value can hide a short, intense burst.
• It is a global average. Kp is based on stations well south of the auroral zone. Locally in Northern Norway, activity can be higher or lower than the number suggests. The Norwegian Centre for Space Weather at UiT (the Arctic University of Norway) therefore shows its own local measurements for places such as Tromsø and Dombås.
• Forecasts lose sharpness quickly. The outlook for the next hour or so is based on measured solar wind. Forecasts one to three days ahead rely on models of what the Sun has sent our way, and both strength and arrival time can be off.
• Kp knows nothing about the weather. Clouds, moonlight, city lights and twilight decide whether you actually see anything. Many good aurora nights are lost under cloud; see clouds and clear skies.

One concrete sign of how much location matters: the Norwegian Centre for Space Weather says that statistically there is some form of aurora over Tromsø every other night, from very faint to very bright. Store norske leksikon likewise notes that inside the auroral zone you get clearly visible aurora on more than half of all nights, even when solar activity is low.

Kp and the clock: UTC or Norwegian time?

NOAA's Kp forecasts are split into three-hour blocks in UTC, for example "18-21UT". Norwegian standard time (CET) is one hour ahead of UTC, and summer time (CEST) two hours ahead; clocks go back on the last Sunday of October. A block labelled 21-00UT therefore means 22:00–01:00 Norwegian time in winter, exactly the hours when the aurora is often most active. More on timing in the best time of night.

In short

• Kp runs from 0 to 9, covers three hours at a time and is a global measure of magnetic disturbance.
• In Northern Norway, Kp 1–3 is often enough in darkness and clear skies. Central Norway needs about 3, the west coast about 4, and the Oslo area about 5 or more.
• G1 equals Kp 5 and G5 equals Kp 9o. Big storms are rare.
• Use Kp as one piece of the puzzle, alongside the cloud forecast, darkness and fresh solar wind data.

If you would rather not do the maths yourself, check tonight's verdict for your location. The app combines Kp, cloud cover and darkness for the spot you choose.