9. Longitudinal Scales-Jets in Transition

This meeting showed that three important transition scales along jets are accessible to observation:

1. A (few-parsec) scale of collimation and acceleration     (see Section 5.1)
2. A (few-kiloparsec, galactic-core) scale of deceleration and recollimation in FR I's (see Section 5.2)
3. A (many-kiloparsec) scale of jet disruption and bending  in FR II's, which (see Norman, these Proceedings) may correspond to that of a self-actuated dentist's drill,  entering the "box of shocklets" it has created in the lobes. This scale of bending  and fraying of FR II jets into filaments  (Clarke, these Proceedings; Hardee, these Proceedings) may be quantifiable in Cygnus A  (Carilli et al., these Proceedings) and in some quasars (e.g.\ 3C175,  Bridle et al. 1994a). 3C353  (Swain, Bridle, & Baum, these Proceedings) may be a case where one jet is presently within its disruption scale and forms a hot spot, while the other is beyond it and flails over the end of its lobe. Might a high-velocity spine  stabilize part of a fraying relativistic jet as the boundary begins to disrupt? And, if many hot spots are transient, formed stochastically off-axis by fraying the jet boundaries, are the most compact hot spots better tracers of the beam paths? (In Cygnus A, the most compact lobe features are closer to the initial jet direction and to the longest axis of the lobe than are the brighter ``classical'' hot spots.)