Most collimated supersonic protostellar jets show a collimated wiggling, andknotty structure (e.g., the Haro 6-5B jet) and frequently reveal a long gapbetween this structure and the terminal bow shock. In a few cases, there is noevidence of such a terminal feature. We present three-dimensional smoothedparticle hydrodynamical simulations which suggest that this morphology may bedue to the interaction of the propagating cooling jet with a non-homogeneousambient medium. In regions where the ambient gas has an increasing density (andpressure) gradient, we find that it tends to compress the cold, low-pressurecocoon of shocked material that surrounds the beam, destroy the bow shock-likestructure at the head, and enhance beam focusing, wiggling, and internaltraveling shocks. In ambient regions of decreasing density (and pressure), theflow 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 couldbe a relic of an earlier outflow episode, as previously suggested, or the placewhere the jet reappears after striking a denser portion of the ambient medium.
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