What is the Kp index and how do you read it?
To make sense of aurora forecasts we use the Kp index — this small integer, running from zero to nine, holds a night's fate in its hand.
The Kp index is a general index graded between the numbers 0 and 9. The “K” index grades the changes that occur in the Earth's magnetic field. 0–1 is the calmest; 5 and above means a geomagnetic storm. The Kp index itself is derived from the data of thirteen observatories lying between latitudes 44 and 66 degrees at the Earth's north and south poles. It is calculated every three hours.
+Added · May 2025·The Bz componentA point I didn't stress when writing the book: Kp alone isn't enough. The Bz component — especially when it turns negative — is the real trigger of the curtain. If this magnetic component of the solar wind points south, surprise outbursts can happen even at a low Kp.×Corrected · October 2024·solar cycle updateIn 2019 this sentence sat at the bottom of solar cycle 24, and it was true. Now we're at the peak of cycle 25: Kp 5+ happens 4–6 times a month. Factor that in when you plan.
A practical reading.
- Kp 0 – 2Only from latitudes 66°+; a greenish glow on the horizon. North of Lofoten, Tromsø.
- Kp 3Tromsø, Abisko and around Reykjavík see it easily. The most common value; most photographs are taken here.
- Kp 4 – 5It droops further south — Oslo, Edinburgh. The colour shifts from green to pink.
- Kp 6 – 9Rare. Word comes from Scotland, even from north of İstanbul. Get outside, no excuses.
descends from the north.
Something more important than the number.
A clear sky. Against a cloudy Kp 7, I'll always take a cloudless Kp 2. The latter will still get you out of your chair for an hour.
+Added · February 2025·Aurora appsThe phone is now part of the kit. Glendale, Hello Aurora, My Aurora Forecast — turn on push notifications in one, and the moment you catch it in the field, tell your partner, tell a friend. It can turn a tired night into a miraculous one.- +Aurora apps box
- ×Solar-minimum sentence · corrected for cycle 25
- +Bz component note
- ○Original · book edition