  We present a new analysis of the physical conditions in three Herbig-Haro
complexes (HH 54, HH 212, and the L 1157 protostellar jet) using archival data
from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. As
described in detail in Paper I, the emission observed using the 4.5-micron
filter is enhanced in molecular shocks (T=1000-4000 K) at relatively high
temperature or densities compared with that observed with the 8.0-micron
filter. Using these data sets, we investigate different distributions of gas
between high and low temperatures/densities. Our analysis reveals the presence
of a number of warm/dense knots, most of which appear to be associated with
working surfaces such as the head of bow shocks and cometary features, and
reverse shocks in the ejecta. These are distributed not only along the jet
axis, as expected, but also across it. While some knotty or fragmenting
structures can be explained by instabilities in shocked flows, others can be
more simply explained by the scenario that the mass ejection source acts as a
"shot gun", periodically ejecting bullets of material along similar but not
identical trajectories. Such an explanation challenges to some degree the
present paradigm for jet flows associated with low-mass protostars. It also
give clues to reconciling our understanding of the mass ejection mechanism in
high and low mass protostars and evolved stars.
