Why is the Kp index used in aurora forecasts?

Answer: It shows geomagnetic unrest

It shows geomagnetic unrest ✓ — Correct! The Kp index is a widely used summary of how disturbed Earth's magnetic field is on a global scale. A higher Kp usually means stronger geomagnetic activity and a better chance that the auroral oval expands away from the poles. That is why many aurora maps and alerts mention Kp: it gives people a quick sense of whether auroras might be visible farther south than usual.

It measures cloud thickness — Wrong. Cloud thickness matters for actual viewing, but Kp is not a cloud or weather measurement. A high Kp can still produce a disappointing night if your sky is overcast.

It counts visible stars — Wrong. The number of stars you can see depends on darkness, cloud cover, and light pollution. Kp describes magnetic disturbance, not star visibility.