  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 is the conversion from magnetic to kinetic energy 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 the region where most of the Doppler boosted radiation observed in blazars
is produced. We briefly discuss the possibility that blazar activity can 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, where acceleration of relativistic particles and production of
non-thermal flares is taking place.
