EXTRACTION OF ROTATING ENERGY FROM A BLACK-HOLE BY A FROZEN-IN MAGNETIC-FIELD IN ACCRETION FLOW

The magnetohydrodynamical structure around a Kerr black hole and the extraction process of the black hole's spin energy has been studied in terms of an exact nonstationary solution for the variation of the magnetic field with a given motion of particles falling radially from rest at infinity. Since the initially homogeneous magnetic field lines are twisted by a frame-dragging effect, the rotational energy of a black hole is stored in the magnetosphere formed around the black hole. In order to examine the possible structure of the magnetic field formed around the hole we precisely investigate the nonstationary magnetic field produced by stationary accretion. A strongly winding field is formed around the event horizon. This horizon is then seen as being wrapped by a bundle of magnetic field lines, though the toroidal component of the field in a comoving frame of particles is in no excess of the radial one. Using the nonstationary solution of the magnetic field, we discuss the dynamical evolution of the energy density distribution of the field as well as energy transfer through the surface of a stretched horizon. The electric energy generation caused by the interaction of an infalling magnetized plasma with the accretion disk is only discussed briefly.