Initial magnetic field. a) Initial B_z at z = 0.05 Mm in grey scale, positive (negative) values are in white (black). The dashed red line indicates the division between the uniform and stretched grid in y. b) Cuts on vertical planes at y = −7.63 Mm (left) and x = −15.14 Mm (right). The magnitude of the magnetic field is shown in grey scale and the black lines correspond the streamlines of the projected magnetic field vector on each plane. c) 3D visualisation of B (thin magenta lines), with the flux rope field lines highlighted (thick magenta lines).