Evolution of the perturbed midplane gas density in the κ_BL-disk simulation. The corresponding simulation time t is given by labels. Individual snapshots represent the state of gas when the total torque acting on the protoplanet is minimal (left), maximal (third), and oscillating in between (second and right). The streamlines are overplotted for reference. The figure is also available as an online movie, showing the temporal evolution from t = 30–33 P_orb.