The KP index is the most widely used measure of global geomagnetic activity — and the most important number for aurora hunters. Here's everything you need to know.
KP stands for "Planetarische Kennziffer" (German for planetary index). It's a 0–9 scale that quantifies disturbances in Earth's magnetic field, averaged across 13 geomagnetic observatories worldwide. A higher number means stronger geomagnetic activity — and a more likely, more vivid aurora.
Every 3 hours, geomagnetic observatories around the globe measure the horizontal component of the local magnetic field. The largest variation from the expected quiet value during each 3-hour window is converted to a K-index (0–9). The 13 worldwide station values are then averaged into the global KP.
The Sun constantly emits charged particles — the solar wind. Coronal mass ejections (CMEs) and high-speed solar wind streams compress Earth's magnetosphere, causing it to "ring" and store energy. When that energy is released into the atmosphere near the poles, it excites atmospheric gases, producing the aurora we see.
Your required KP depends on your latitude. The higher your latitude, the lower the KP threshold for visible aurora. Below are the typical minimum KP values for common destinations.
KP is a useful starting point, but it's a single global number measured every three hours — it says nothing about whether the sky above you will actually be clear, how dark it is at your specific location, or where the auroral oval sits at the moment you're looking up. Two nights with the same KP value can look completely different on the ground.
That's why PolarForecast combines KP with the position of the auroral oval, real-time geomagnetic activity (including Bz and solar wind), cloud cover, darkness, and your exact location — rather than showing you a single number and leaving the rest to guesswork. See our forecasting methodology for how these factors combine. Want to understand the number itself in more depth? Read our full KP index guide.