The lights overhead are the last step in a chain that starts on the surface of the sun, three to four days earlier. A coronal mass ejection throws a cloud of charged particles outward; if it’s aimed roughly at Earth, those particles reach our magnetosphere and get funneled toward the poles along magnetic field lines.
Our forecasting lead watches solar imagery for exactly these ejections, days before any KP-index tool would show anything at all — by the time an aurora app lights up green, we’ve usually already adjusted the week’s itinerary around it.