3D density plot, measured in 10^-12 kg/m³, of our basic setup (t = 0), and of the three different models at later times. The cross sections on the x − y planes at the footpoint (z = 0) and the apex (z = L/ 2 = 100 Mm) are shown. The region with the highest refinement level is defined by 0 ≤ z ≤ 100 Mm, | x | ≤ 2.33 Mm and y ≤ 2.33 Mm. Animations of these figures, showing the oscillations for the three models, are available online (movies 1–3).