The Sun constantly streams charged plasma outward. Earth's magnetic field deflects most of it into a teardrop-shaped magnetosphere — but when the wind is fast, dense, and the interplanetary field tips southward, field lines on the dayside reconnect and plasma pours in at the poles, lighting up the aurora.
💡 Kp / aurora-latitude figures below use a simplified illustrative model (loosely following the Newell coupling function), not a live space-weather forecast.