Vortex streamline at t = 300 and t = 1000: the shape of the vortex evolves during the simulation. The radial extent of the vortices is constant and fixed by the radial extent of the density bump, while their azimuthal extent increases when the m mode number decreases. In the right figure, the streamlines become elliptic and an aspect ratio can be estimated (~6 here, but each vortex has a different aspect ratio, as can be seen in Fig. 3).