Formation of bipolar regions for three different stratifications (left column: A3, middle: A5, right: A7). Top row: normalized vertical magnetic field B_z/B_eq plotted at the xy surface (z = 0) at times when the bipolar regions are the clearest. Second row: normalized magnetic energy density plotted in the yz plane as a vertical cut through the bipolar region at x = 0. We replicated the domain by 50% in the y direction (indicated by the vertical dashed lines) to give a more complete impression about spot separation and arch length. The black-white dashed lines indicate the replicated part and in the last three rows the surface (z = 0). Third row: vertical rms magnetic field normalized by the local equipartition value (see Fig. 1 for vertical profiles) as a function of t/τ_td and z/H_ρ. Bottom row: smoothed effective magnetic pressure as a function of t/τ_td and z/H_ρ. Blue shades correspond to negative and red to positive values.