Overall magnetic field line topology before the first eruption t = 274 min ((a) side view, (b) top view). The white crosses indicate the locations at which the current sheets are formed. Panels c and d: focus on the upper part of the field line system. Blue field lines are part of the magnetic field lobes traced around z = 13 Mm. The grey field lines result from the reconnection between blue field lines. Panels e and f: lower part of the magnetic field line system. Pink field lines are lower-lying sheared field lines of the magnetic lobes. The orange field lines result from the reconnection between pink field lines. The blue-red patches at the photospheric plane show the negative-positive B_z component of the magnetic field saturated at ±300 G.