in @DeVore91. The time-dependent equations of ideal MHD, with the magnetic forces expressed in the Lorentz form, are solved on a dynamically solution-adaptive grid managed by the toolkit [*PARAMESH*]{} [@MacNeice00]. This grid refines and derefines adaptively during the simulation as prescribed in the Appendix of @Karpen12. A Cartesian domain is assumed with $x$ and $y$ the horizontal axes and $z$ the vertical axis. The simulation domain spans $[-12L_0;12L_0]\times[-12L_0;12L_0]\times[0;24L_0]$. The nonuniform initial grid is identical to the one presented in Fig. 1 of PAD09. The same boundary conditions are used as in PDD15, i.e. line-tied at the bottom, closed on the four sides, and open at the top.

The initial magnetic field is set to be potential, using the specific analytical form given in Sect. 3 of PDD15. The initial configuration (cf. top-left panel of Fig. \[FigEvolBeta0025\]) is given by a central vertical magnetic dipole placed under the photosphere (producing a locally closed coronal field, e.g., purple lines in Fig. \[FigEvolBeta0025\]), embedded in an inclined (with respect to the vertical direction) and uniform background magnetic field (the open field, e.g., pink lines in Fig. \[FigEvolBeta0025\]). A 3D null point with its associated fan surface and two spine lines are present, with 