Ghost-cell exchange for blocks (a) at the same resolution level and for coarse-fine interfaces in (b) restriction and (c) prolongation. The numbers represented are the local grid indices to identify neighbors and ghost regions, as explained in Sect. 4.4. Lines drawn in color denote (magnetic) fluxes on the boundary faces that are communicated. Panel c: dashed lines denote boundaries that contain only tangential fluxes necessary for restriction which reside only on the coarse buffer of a block. The size of fine blocks including ghost regions is marked by a gray line.