From left to right and top to bottom: snapshot of the relative vorticity perturbation at the 1087th, 1124th, 1173th, 1208th, 1216th, 1225th, 1228th, and 1310th orbit. The relative vorticity perturbation has been vertically averaged. We draw with a black line the iso-contour corresponding to 80% of the density maximum ρ_m. The location of the density maximum is shown by the black cross.