The Kp index is a single number, from 0 to 9, that sums up how disturbed Earth's magnetic field is right now. The higher it climbs, the further from the poles the aurora can be seen - which is exactly why it's the first number every aurora-watcher checks.
The short answer
"Kp" stands for planetary K-index - short for the German "planetarische Kennziffer", introduced by Julius Bartels in 1949. Every three hours, magnetic observatories around the world measure how much the local magnetic field has been disturbed by the solar wind. Those readings are combined into one global figure: the Kp index.
A quiet night sits at Kp 0–2. A decent display reaches Kp 4–5. The rare, history-making storms that paint aurora over southern Europe are Kp 8–9. Because it's measured globally, the same Kp number applies everywhere on Earth at once - what changes is what that number means for your latitude.
Quick rule of thumb: the further you are from the magnetic poles, the higher the Kp you need. Tromsø sees aurora at Kp 1; London needs roughly Kp 6–7.
The 0–9 scale
Each step up the scale roughly doubles the disturbance. Here's what each band means and how far south the lights typically reach:
How Kp is calculated
Thirteen observatories at mid-latitudes record how far Earth's magnetic field deviates from its quiet-day baseline every three hours. Each station converts its measurement to a local K index (0–9), then the 13 readings are averaged and standardised into the global Kp value.
The logarithmic nature of the scale matters. A jump from Kp 5 to Kp 7 is not twice as strong - it represents roughly 10 times the field disturbance. This is why aurora expands dramatically southward during high-Kp events.
What Kp doesn't tell you
Kp is powerful but blunt. It's an average over three hours and the whole planet, so it misses a lot of what decides your actual night:
- Cloud cover - a clear Kp 3 beats a cloudy Kp 6 every time.
- Timing - Kp can spike for 20 minutes and the three-hour value barely moves.
- The Bz direction - when the solar wind's magnetic field points south, energy pours into the magnetosphere. This often matters more than Kp in the moment.
That's why the aurora forecast on this site pairs Kp with live solar-wind data, local cloud cover, and moon phase - turning one blunt number into a real answer for your sky.
Aurora by Kp level
Each Kp level reaches a different latitude band. Select a Kp level to see all locations that become visible at that threshold:
