Abstract
There is a growing recognition that magnetic fields play a dominant role in driving and collimating astrophysical jets. Using exact semianalytic solutions of the special-relativistic ideal-MHD equations, it is demonstrated that a strongly magnetized accretion disk (with a dominant poloidal or azimuthal magnetic field) around a black hole can efficiently accelerate a proton-electron outflow to the Lorentz factors and kinetic energies inferred in gamma-ray burst sources and in active galactic nuclei. The possible contributions of thermal driving (by the pressure of radiation and electron-positron pairs) and of nonideal-MHD effects to the acceleration of the flow are also discussed. It is pointed out that MHD acceleration is distinguished by being spatially extended and that this property should be most noticeable in relativistic flows. It is argued that observational indications that ``superluminal'' radio jets are accelerated over distances that far exceed the scale of the central black hole support the magnetic-driving picture. Preliminary results of a comprehensive model that could be used to test this interpretation are presented.
