Live aurora forecast
Reading the Kp index, the bar colours and the solar wind
Chapter 2 of the northern lights course. The Kp index is one number from 0 to 9 that says how far the aurora reaches tonight. This chapter turns it into a decision: enough for your place or not, and what the six live figures on the dashboard add.
In short
The Kp index is a 0 to 9 scale of how disturbed Earth's magnetic field was in the last 3 hours, published by NOAA from magnetometers around the world. Higher Kp means the auroral oval reaches further from the poles: Tromsø needs Kp 1, Rovaniemi 2, Edinburgh 4, Kiel 6 and Spain 8 or 9. The bar colours on this site follow NOAA's words: quiet, unsettled, active, then storms G1 to G5.
Key points
- Kp runs from 0 to 9 in 3-hour intervals and is published by NOAA
- Each Kp level moves the edge of the oval about 2 degrees toward the equator
- Green on the bar is quiet, yellow unsettled, orange active, red G1 and G2, purple G3 to G5
- Solar wind speed, density and Bz say what the next hour may bring
- A negative Bz is the door opening: the single most useful live figure
On this page
1What the Kp index measures and where it comes from
Kp stands for planetary K index. Every three hours, magnetometers at stations around the world record how much the ground-level magnetic field wobbled, each station's reading is turned into a K value from 0 to 9, and the planetary average becomes the Kp. NOAA SWPC publishes it in thirds (3.33, 3.67) and shows it on this site's dashboard as the big number. A high Kp does not mean a bright aurora at your spot. It means the auroral oval has expanded, so places further from the magnetic pole come inside it.
That is the whole trick for a traveller. The question is never whether Kp 4 is good in general. It is whether Kp 4 is enough for the place you are standing. NOAA gives the rule: the edge of the oval sits at about 66 degrees geomagnetic latitude at Kp 0 and moves about 2 degrees toward the equator per Kp level. Geomagnetic latitude is measured from the magnetic pole, not the geographic one, which is why Yellowknife, at a lower latitude than Tromsø, needs the same Kp.
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Frequently asked questions
What Kp index do you need to see the northern lights?
It depends on the place. Under the oval, Tromsø, Iceland or Yellowknife, Kp 1 is enough on a dark clear night. Rovaniemi needs Kp 2, Edinburgh 4, Kiel 6 and Spain 8 or 9. Each level moves the edge of the oval about 2 degrees toward the equator, by NOAA's rule of thumb.
What do the colours on the Kp index bar mean?
They follow NOAA's words for the scale: green is quiet (Kp 0 to 2), yellow unsettled (3), orange active (4), red the minor and moderate storms G1 and G2 (5 and 6) and purple the strong to extreme storms G3 to G5 (7 to 9). Hovering a cell on the dashboard lists the places that level reaches.
Is a Kp index of 3 good for the northern lights?
Yes inside and near the oval. Kp 3 is NOAA's unsettled level and brings Rovaniemi, Anchorage and Shetland into reach, while Tromsø and Iceland get a bright night. It is not enough for Edinburgh, Kiel or anywhere further south.
Why does the Kp index change only every three hours?
Because that is how it is defined: magnetometers around the world record the disturbance over a 3-hour interval, each station gets a K value from 0 to 9 and the planetary average is the Kp. NOAA also publishes an estimated value for the current interval, which is what the dashboard shows as Kp now.
What does a negative Bz mean for the Kp index and the northern lights?
That the magnetic field carried by the solar wind points south, opposite to Earth's field, so the two connect and energy pours into the magnetosphere. As a rule of thumb among aurora chasers, not a NOAA threshold, a Bz around minus 5 nanotesla makes a night interesting and below minus 10 storms build. With a positive Bz even fast, dense wind does little.
Does a high solar wind density guarantee a high Kp index and northern lights?
No. Density, a few protons per cubic centimetre on an ordinary day, measures how many particles are pushing on the field. A jump signals an arriving shock, but the aurora still needs a southward Bz to let the energy in, and a dark, clear sky to be seen. The three figures are read together.
What is the difference between a NOAA watch, warning and alert on the Kp index?
A watch is issued days ahead when a storm is expected, usually after a coronal mass ejection leaves the Sun. A warning is issued hours ahead. An alert says the Kp threshold has been crossed and the storm is happening now. The dashboard shows the latest one in force with its time.
Sources (5)
Sources
- NOAA SWPC, planetary K-index, checked on 9 Oct 2026
- NOAA SWPC, NOAA space weather scales, checked on 9 Oct 2026
- NOAA SWPC, tips on viewing the aurora, checked on 9 Oct 2026
- NOAA SWPC, Solar wind, checked on 9 Oct 2026
- NOAA SWPC, real-time solar wind, checked on 9 Oct 2026
Written and maintained by the Kp Forecast editorial team. Independent site, not a space weather agency. Data: NOAA SWPC.
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