Buoyancy and Penrose Process Produce Jets from Rotating Black Holes
  The exact mechanism by which astrophysical jets are formed is still unknown. It is believed that necessary elements are a rotating (Kerr) black hole and a magnetised accreting plasma. We model the accreting plasma as a collection of magnetic flux tubes/strings. If such a tube falls into a Kerr black hole, then the leading portion loses angular momentum and energy as the string brakes, and to compensate for this loss, momentum and energy is redistributed to the trailing portion of the tube.} {We found that buoyancy creates a pronounced helical magnetic field structure aligned with the spin axis. Along the field lines, the plasma is centrifugally accelerated close to the speed of light. This process leads to unlimited stretching of the flux tube since one part of the tube continues to fall into the black hole and simultaneously the other part of the string is pushed outward. Eventually, reconnection cuts the tube, the inner part is filled with new material and the outer part forms a collimated bubble-structured relativistic jet. Each plasmoid can be considered like an outgoing particle in the Penrose mechanism: it carries extracted rotational energy away from the black hole while the falling part with the corresponding negative energy is left inside the ergosphere. 
V.S. Semenov, S.A. Dyadechkin, M.F. Heyn
astro-ph.HE