BP1–ER1. Left panel: loop structure of the BP1 region and the neighbouring smaller loop system that host the first observed eruption. The bottom plane represents the magnetogram scaled to ±85.3 Gauss. The field lines are colour-coded with a non-dimensional magnitude of the current. The current amplitude is in units of 0.0188 Amperes. The right panel: additionally includes the large-scale magnetic field (blue field lines) that embeds the loop systems seen in the left panel. Time: 22:39 UT. An animation (BP1_erupt1.mov) associated to the left panel is available online.