Simulated field lines of flux rope system identified from the current density, squashing factor and twist maps at the x = 76 Mm plane on 22 April 2013 at 08:36 UT, just before the onset of the observed eruption at from 08:40 UT onwards (Sect. 2.2). Panel a: main FR field lines viewed from the southeast. The lines are rainbow-colored based on the seed point position of the lines at the x = 76 Mm plane. The B_z magnetic field component is plotted at the photospheric z = 0 lower boundary of the simulation domain in black-white coloring saturated at ±100 Mx cm⁻² (=0.01 T). Panel b: intertwined FR with yellow-purple coloring. Panel c: both FRs viewed from the northwest. All plotted field lines are based on finding a coherent region on the x = 76 Mm plane where the condition [(log₁₀Q ≤ 1.5)∧(T_w ≤ −1)] is fulfilled. Animated versions of each panel can be found online. For panels a and b, online movies 3 and 4 also offer the view from the northwest, as in panel c.