Vertical magnetic field at the photosphere at both loop footpoints at the depth where ⟨τ⟩=1. The snapshot was taken at t = 24.34 min. The red arrows show the magnitude of the horizontal velocity at the photospheric level (see legend at top right). The blue contours indicate regions with |B_z|> 1000 G. The green triangles mark the footpoints of the fieldline investigated in Sect. 3.2.2. For a discussion of the initial condition for the magnetic field see Sect. 2.3. A movie is available online.