Pre-eruptive MFR (yellow magnetic field lines) traced from a small sphere centred at and spanning a quasi-circular concentration of enhanced J_par outlined with dashes in the xz-midplane. The J_par distribution in the xz-midplane is translated from its original position for clarity. The purple magnetic field lines are traced from the same seeds as the field lines shown in Fig. 8. We display the photospheric B_z distribution with a greyscale. The figure corresponds to time-step 53.