Are Quasar Jets Matter or Poynting Flux Dominated?
  If quasar jets are accelerated by magnetic fields but terminate as matter dominated, where and how does the transition occur between the Poynting-dominated and matter-dominated regimes? To address this question, we study constraints which are imposed on the jet structure by observations at different spatial scales. We demonstrate that observational data are consistent with a scenario where the acceleration of a jet occurs within 10^{3-4} R_g. In this picture, the non-thermal flares -- important defining attributes of the blazar phenomenon - are produced by strong shocks formed in the region where the jet inertia becomes dominated by matter. Such shocks may be formed due to collisions between the portions of a jet accelerated to different velocities, and the acceleration differentiation is very likely to be related to global MHD instabilities. 
Marek Sikora (CAMK), Greg M. Madejski (SLAC, KIPAC), Jean-Pierre   Lasota (IAP) and Mitchell C. Begelman (JILA)