Annu. Rev. Astron. Astrophys. 1988. 36: 539-598
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4.5. Astrophysical Applications and Comments

The general conclusion from the above simulations is that shear instabilities are crucial in the propagation of supersonic jets. Nonlinear effects may stabilize magnetic and/or dense jets, but light hydrodynamic jets are prone to disruption by turbulent mixing on time scales that are short with respect to observed scales. Observations do in fact point toward the light jet case. The solution seems to be the reduction of the interaction between the jet and the external medium, either by magnetic fields or the presence of extended boundary layers or surface currents.

The application of the above results to classes of radio galaxies predicts that, if jets have similar initial Mj, FR I jets, which are turbulent and strongly decelerated, should correspond to dense environments (light jets), and FR II to lighter environments. The morphology of jets would depend on the ambient density, which is in agreement with a suggestion by De Young (1993). However, Mj may also have an influence. In particular, for a similar density ratio, highly supersonic jets tend to be less turbulent (FR II sources), and mildly supersonic jets more turbulent (FR I sources). These results fit in the unification schemes discussed in Section 2.2.