Jet dynamics. Recollimation shocks and helical patterns
  The dynamics and stability of extragalactic jets may be strongly influenced
by small (and probable) differences in pressure between the jet and the ambient
and within the jet itself. The former give rise to expansion and recollimation
of the jet. This occurs in the form of conical shocks, or Mach disks, if the
pressure difference is large enough. Pressure asymmetries within the jet may
trigger the development of helical patterns via coupling to kink current-driven
instability, or to helical Kelvin-Helmholtz instability, depending on the
physical conditions in the jet. I summarize here the evidence collected during
the last years on the presence of recollimation shocks and waves in jets. In
the jet of CTA 102 evidence has been found for (traveling)shock-(standing)shock
interaction in the core-region (0.1 mas from the core), using information from
the light-curve of the source combined with VLBI data. The conclusions derived
have been confirmed by numerical simulations combined with emission
calculations that have allowed to study the spectral evolution of the perturbed
jet. Helical structures can also be identified in radio-jets. The ridge-line of
emission of the jet of S5~0836+710 has been identified as a physical structure
corresponding to a wave developing in the jet flow. I review here the evidence
that has allowed to reach this conclusion, along with an associated caveat.
Current data do not allow to distinguish between magnetic or hydrodynamical
instabilities. I finally discuss the importance of these linear and non-linear
waves for jet evolution.
