On Why Disks Generate Magnetic Towers and Collimate Jets
  We show that accretion disks with magnetic fields in them ought to make jets provided that their electrical conductivity prevents slippage and there is an ambient pressure in their surroundings.   We study equilibria of highly wound magnetic structures. General Energy theorems demonstrate that they form tall magnetic towers whose height grows with every turn at a velocity related to the circular velocity in the accretion disk.   The pinch effect amplifies the magnetic pressures toward the axis of the towers whose stability is briefly considered. 
D Lynden-Bell, (The Observatories of the Carnegie Institution,   Pasadena, CA., U.S.A. The Institute of Astronomy, Universit...