2005ApJ...625...72S

The formation of relativistic astrophysical jets is presumably mediated by magnetic fields threading accretion disks and central, rapidly rotating objects. As it is accelerated by magnetic stresses, the jet's kinetic energy flux grows at the expense of its Poynting flux. However, it is unclear how efficient the conversion from magnetic to kinetic energy is and whether there are any observational signatures of this process. We address this issue in the context of jets in quasars. Using data from all spatial scales, we demonstrate that in these objects the conversion from Poynting flux-dominated to matter-dominated jets is very likely to take place closer to the black hole than in the region where most of the Doppler-boosted radiation observed in blazars is produced. We briefly discuss the possibility that blazar activity could be induced by global MHD instabilities, e.g., via the production of localized velocity gradients that lead to dissipative events such as shocks or magnetic reconnection, in which acceleration of relativistic particles and production of nonthermal flares is taking place.