Role of surface modes in vortex formation in BEC

We study the role of surface modes in the process of vortex formation in harmonically trapped BEC. It is shown that the vortex nucleation and penetration to the inner part of the cloud occur at velocities slightly exceeding the surface mode critical velocity. Surface modes induce ripples of the order parameter; these ripples are then transformed into vortex-antivortex pairs. After this, vortices move to the inner part of the cloud, whereas antivortices go in the opposite direction