Force-Free Approximation — the Black Hole Magnetosphere

Abstract

AbstractThis chapter mainly deals with the magnetosphere structure of supermassive black holes for which the magnetic fields are of greater importance than for galactic solar mass black holes. Therefore, it is advisable to discuss the problems of the magnetic field generation in accretion disks in this chapter. We also briefly discuss the effect of the magnetic field on the disk accretion and matter outflow processes. Since the black hole itself cannot have a self-magnetic field (so-called ‘‘no hair’’ theorem), the large-scale magnetic field in the vicinity of the black hole can be generated only in the accretion disk. The missing link that helps one understand the energy release mechanism, which effectively transfers the energy from the rotating black hole and/or the inner parts of the disk to the active regions, is, in most cases, exactly associated with this regular magnetic field and the collimation mechanism responsible for the jet formation. In other words, to explain the galactic nuclei activity we use the pulsar idea of a unipolar inductor, the work of which is just provided by the regular poloidal magnetic field, the central object rotation (giving rise to the induction electric field E), and the longitudinal electric current (giving rise to the toroidal magnetic field B φ). Within this model, the energy flux, as in the case of the radio pulsar magnetosphere, is fully connected with the electromagnetic energy flux, and the jet formation is, ultimately, due to the known property of the parallel current attraction. Thus, the main constituent elements of the central engine are a supermassive black hole, the accretion disk, and the regular magnetic field. On the basis of this model, the physical nature of the Blandford–Znajek mechanism is discussed in detail. Finally, the exact analytical solutions obtained by the force-free approximation are discussed.KeywordsBlack HoleNeutron StarEvent HorizonAccretion DiskActive Galactic NucleusThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.