Annu. Rev. Astron. Astrophys. 1988. 36: 539-598 Copyright © 1998 by Annual Reviews. All rights reserved

2. THE PHENOMENOLOGICAL MODEL

Because VLBI measurements of proper motions in the compact regions of jet formation favor flow velocities that are very close to the speed of light, it appears that jet acceleration is a highly relativistic process that takes place in the vicinity of a gravitational horizon. In addition, since a high degree of collimation is observed in jets, confinement is required. The most obvious agent is pressure by an external medium and/or magnetic field. Typical plasma parameters of the ambient surrounding radio galaxies are listed in Table 3.

The measurements of the ambient pressures come from the low angular resolution X-ray surveys of the regions around radio jets, but they are still relatively scarce. Structures in strong jets appear to be often overpressured (Bicknell & Begelman 1996), while weak jets are always underpressured or in pressure equilibrium (Feretti et al 1995). Obviously, observed features may in fact be transients.

The standard synchrotron plasma model requires that radio-emitting regions contain a suprathermal electron gas that coexists with a cold component (protons and nuclei) that dominates the mass content. Some authors have suggested that outflows are made of electron-positron pairs (with and without a proton-electron component) in order to explain the fact that energy is not deposited along jets and to reduce the kinetic energy content in radio lobes (Bicknell & Begelman 1996, Kundt 1996).

A few considerations are worth mentioning in connection with this picture. The first is about the observed morphologies of magnetic fields, as derived from polarized emissivity measurements. Continuous emission comes from discrete structures as knots and filaments that may not be in steady pressure equilibrium, and they may not even be in equipartition conditions but transient. In addition, they lower the emission filling factor and reduce the energy budget.

Second, the early proposal of an adiabatic expansion of emitting structures (blobs) along the jet that would make emissivity critically dependent on the source projected cross section [ r^-(5+4), spectral index] is not verified by observations (Scheuer 1974). Thence either turbulent amplification of magnetic fields occurs everywhere, or transient formations dominate the dynamics.