A Possible Mechanism for Wiggling Protostellar Jets from 3D Simulations   in a Stratified Ambient Medium
  Most collimated supersonic protostellar jets show a collimated wiggling, and knotty structure (e.g., the Haro 6-5B jet) and frequently reveal a long gap between this structure and the terminal bow shock. In a few cases, there is no evidence of such a terminal feature. We present three-dimensional smoothed particle hydrodynamical simulations which suggest that this morphology may be due to the interaction of the propagating cooling jet with a non-homogeneous ambient medium. In regions where the ambient gas has an increasing density (and pressure) gradient, we find that it tends to compress the cold, low-pressure cocoon of shocked material that surrounds the beam, destroy the bow shock-like structure at the head, and enhance beam focusing, wiggling, and internal traveling shocks. In ambient regions of decreasing density (and pressure), the flow widens and relaxes, becoming very faint. This could explain ``invisible'' segments in systems like the Haro 6-5B jet. The bow shock in these cases could be a relic of an earlier outflow episode, as previously suggested, or the place where the jet reappears after striking a denser portion of the ambient medium. 
E.M. de Gouveia Dal Pino, M. Birkinshaw, and W. Benz