A Cosmic Battery around Black Holes

Abstract

Energetic astrophysical jets have always been associated with relatively strong large-scale magnetic fields that extract energy from the rotation of a central black hole and its surrounding accretion disk. This natural association does not answer the fundamental question what is the origin of the large-scale magnetic field. When the disk is non-diffusive, standard MHD advection can bring the field in from large distances, as attested by the multitude of ideal MHD simulations of magnetized astrophysical accretion disks. When this is not the case, however, the large-scale dipolar magnetic field expected at the origin of astrophysical jets is generated naturally by the anisotropic radiation pressure around the central black hole. In this Chapter, we discuss the scenario where the innermost part of the accretion disk generates and holds one polarity of the magnetic field, while the return polarity diffuses outward through the outer diffusive part of the disk. This is the Cosmic Battery first proposed by Contopoulos and Kazanas in 1998.