Kp Index
Definition
A 0–9 scale summarizing how disturbed Earth's magnetic field is worldwide over three-hour periods.
In plain language
Kp condenses readings from magnetometers around the world into one number. Higher Kp means a more disturbed field and an auroral oval pushed farther from the poles — which is why aurora hunters quote it constantly.
The science
Introduced by Julius Bartels in 1939, Kp averages standardized K-indices from a global station network per 3-hour UT interval, on a quasi-logarithmic scale in thirds (e.g. 4-, 4o, 4+). Kp ≥ 5 defines geomagnetic storm levels.
Why it matters for aurora viewing
Match Kp to your magnetic latitude: Kp 2 suffices in Fairbanks, Kp 5 for the northern US, Kp 7+ for the mid-latitudes. It's a planning tool, not a light switch.
See this in action on tonight's data.