Temperature distribution (upper panel) and vertical magnetic field B_z (lower panel) along the cut through the FT indicated in Fig. 2 (diagonal line). The colored dots indicate several τ = 1 levels (white: continuum near 525 nm; yellow: 525.02 nm line core; brown: 630.15 nm line core). We note that the values of the x-axis correspond to those of the x-axis of the simulation and not to the length along the diagonal cut. I.e., the true distance between two points along the (diagonal) cut is longer by a factor .