Dynamical behaviour of the 'beads' along the magnetic field lines near a rotating black hole

The elements of cold, magnetically driven flows behave like beads on the magnetic field lines. The inclination of the field lines at the surface of the disc plays a crucial role in the nature of the magnetically driven outflow. For the non-relativistic case, a centrifugally driven outflow of matter from the disc is possible if the poloidal component of the magnetic field makes an angle less than a critical 60 degrees with the disc surface. The collimated flows ejected from active galactic nuclei probably start from the region near the black hole. We investigate the dynamical behaviour of the 'beads' on the magnetic field lines, starting from the disc near a black hole. It is found that the critical angle becomes larger than 60 degrees for the rotating black hole case, which may imply that the flows are easily accelerated in the inner edge of the disc surrounding a rotating black hole.