Annu. Rev. Astron. Astrophys. 1988. 36: 539-598 Copyright © 1998 by Annual Reviews. All rights reserved

3.2. Electromagnetic Winds from Black Hole/Disk Magnetospheres

Blandford (1976), Lovelace (1976) were first to discuss the generation of electromagnetic winds from force-free magnetospheres above thin accretion disks. They found that the rotation of the disk causes the magnetic lines to sweep the ambient plasma, which consequently feels a strong induced poloidal electric field. Correspondingly, as the magnetic field that is predominantly toroidal at large distances has a poloidal component, momentum and energy are carried away by an electromagnetic Poynting flux. The poloidal component of this flux lies on paraboloidal surfaces, and consequently, energy is focused on the rotation axis and carried away along it as an electromagnetic wind. In addition, a closed electric circuit is set up with a radial current flowing outward in the disk plane and an axial current flowing in along the rotation axis. This axial current can be initiated by a flow of relativistic electrons. At large distances, an ambient plasma can absorb the Poynting flux and give rise to particle jets. The ultimate energy source is rotation.

An important class of electromagnetic models considers Kerr black holes immersed in a large-scale magnetic field maintained by external sources (e.g. by currents in an accretion disk), whose flux is assumed to thread in part the event horizon. The basic picture is that matter inflowing onto the hole with large angular momentum carries a component B_z of a magnetic field parallel to the rotation axis. The configuration becomes analogous to the unipolar inductor proposed by Goldreich & Julian (1969) for pulsars and supports ejection of Poynting flux and relativistic plasma along open field lines (Phinney 1982).

Blandford & Znajek (1977), Macdonald & Thorne (1982), Phinney (1982) examined a model of rotational energy extraction from Kerr black holes looking for a direct link with the properties of black holes. A way to extract this energy is to assume that magnetic flux lines threading the event horizon originate from an electric potential difference between the poles and equator of the hole that causes a current to flow. For a 10⁸ M hole and a 10⁴ G magnetic field, this potential difference can be as large as 10²⁰ V. So large potentials do allow the production of electron-positron pairs by vacuum breakdown and maintain currents ~ 10¹⁸ Amp from the horizon to infinity. A steady-state solution has been derived by Phinney (1982) in which two winds emanate from a source region inside the magnetosphere: a wind of charges falling into the hole and another moving outward. This last can be seen as a relativistic MHD wind where energy is mostly transported by Poynting flux. The main difference between this model and those of Blandford and Lovelace is that a black hole is a very good conductor with an electric resistance of ~ 100 ohm. The magnetic coupling between the hole and the magnetosphere extracts work from the hole that is equal to the back-reaction from the magnetosphere to the hole plus the ohmic dissipation in the external load (magnetosphere and disks). The model is still controversial. Punsly & Coroniti (1990) have pointed out that the ingoing wind must flow faster than any MHD wave signal, and therefore the event horizon is fundamentally without a causal contact with the source region and the outgoing wind itself. Blandford (1989) has proposed that the causal connection can be established by the gravitational dragging of the reference frame, but the issue is still not clarified. However, energy extraction from black holes via Poynting flux appears to be a very promising solution that overcomes the problems related to the transparency of the deep cores of AGNs to relativistic matter.