Northern lights

Three things must line up at once: activity (Kp must reach the threshold for your location), darkness (the sun far enough below the horizon) and clear sky (no clouds). The further north, the lower the Kp needed — in Kiruna Kp 1 is often enough, in Stockholm ~Kp 5 is required, in Malmö ~Kp 6–7. The aurora season in Sweden is roughly September–March; in summer the nights are too bright even when activity is high.