Comparison of u_∥/c₀ from the original simulation (left) and the Bifrost simulation (right) of at t = 58.5 s. The colour range is the same for the two figures. The thin black lines indicate random magnetic field lines where the two thick black lines indicate the magnetic field lines attached to the edges of the piston. The thin white lines indicate iso-plasma β contours labelled by their respective values. The thick white lines indicate the layer where β = 1. The red lines are reference lines, at (horizontal) z = 499 km, z = 933 km and (vertical) x = 3.81 Mm.