2003A&A...408..415H

The powerful and highly collimated jets observed in active galactic nuclei and µ-quasars are likely to be connected to the accretion phenomenon via disks. Based on theoretical arguments and quasi-stationary radiative MHD calculations, a model for an accretion-powered jet is presented. It is argued that accretion disks around black holes consist of 1) a cold, Keplerian-rotating and weakly magnetized medium in the outer part, 2) a highly advective and turbulent-free plasma inside r_tr=10-20 Schwarzschild radii, where magnetic fields are predominantly of large scale topology and in excess of thermal equipartition, and 3) an ion-dominated torus in the vicinity of the hole, where magnetic fields undergo a topological change into a monopole like-configuration. The action of magnetic fields interior to r_tr is to initiate torsional Alfven waves that extract angular momentum from the disk-plasma and deposit it into the transition layer between the disk and the overlying corona, where the plasma is dissipative and tenuous. A significant fraction of the shear-generated toroidal magnetic field reconnects in the transition layer, thereby heating the plasma up to the virial-temperature and forming a super-Keplerian rotating, and hence centrifugally accelerated outflow. The strong magnetic field in the transition layer forces the electrons to cool rapidly which, in combination with the fast outward-oriented motion, yields a two-temperature ion-dominated outflow. The toroidal magnetic field in the transition layer is in thermal equipartition with the ions, whereas the poloidal component is in equipartition with the electrons. Such a strong toroidal magnetic field is essential for increasing the jet-disk luminosity in the radio regime. These gravitationally unbound outflows serve as seeds, possibly, for all the powerful electron-proton jets observed in accreting systems containing black holes.