The remarkable AGN jets
  The jets from active galactic nuclei exhibit stability which seems to be far
superior compared to that of terrestrial and laboratory jets. They manage to
propagate over distances up to a billion of initial jet radii. Yet this may not
be an indication of some exotic physics but mainly a reflection of the specific
environment these jets propagate through. The key property of this environment
is a rapid decline of density and pressure along the jet, which promotes its
rapid expansion. Such an expansion can suppress global instabilities, which
require communication across the jet, and hence ensure its survival over huge
distances. At kpc scales, some AGN jets do show signs of strong instabilities
and even turn into plumes. This could be a result of the flattening of the
external pressure distribution in their host galaxies or inside the radio
lobes. In this regard, we discuss the possible connection between the stability
issue and the Fanaroff-Riley classification of extragalactic radio sources. The
observations of AGN jets on sub-kpc scale do not seem to support their supposed
lack of causal connectivity. When interpreted using simple kinematic models,
they reveal a rather perplexing picture with more questions than answers on the
jets dynamics.
