The Kp index is the planetary measure of geomagnetic disturbance, from 0 (quiet) to 9 (extreme storm), updated every three hours from a global magnetometer network. Aurora chasers, satellite operators and grid engineers all read the same number.
Understanding Kp Index
From magnetometer trace to planetary number
Each contributing observatory scales its field disturbance into a local K index (0–9) calibrated so its latitude's typical storm response uses the full range; the planetary algorithm weights and averages the network into Kp. The quasi-logarithmic design means each step up roughly doubles disturbance energy — Kp 7 is not "two better" than 5 but several times stronger — a nuance forecasts routinely have to explain.
Kp in satellite operations
High Kp reshapes the thermosphere within hours: Joule heating swells atmospheric density at orbital altitudes, multiplying drag on everything below ~600 km. Operators see it as sudden along-track errors against predictions, swollen TLE residuals, and — after major storms — days of catalogue recovery while orbits are re-fitted. Conjunction screening widens its uncertainties accordingly, and re-entry predictions during storms carry the largest error bars of all.