Kp index
Also called: Kp, geomagnetic storm.
A 0–9 scale of how disturbed Earth's magnetic field is — the higher, the farther from the poles aurorae reach.
Kp 0–2 is quiet; Kp 5 is a minor geomagnetic storm, when aurorae can reach about 55–60° latitude; Kp 7–9 storms bring them to 45° or lower. NOAA's Space Weather Prediction Center forecasts it a few days ahead.