  In this paper we present the first SiO multiline analysis (from J=2-1 to
J=11-10) of the molecular bullets along the outflows of the Class 0 sources
L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. We
have computed the main physical parameters in each bullet and compared them
with other tracers of warm and dense gas and with models for the SiO excitation
in shocks. We find that the bullets close to L1448--mm, associated with high
velocity gas, have higher excitation conditions (n(H2) ~ 10^{6} cm^{-3}, T >
500 K) with respect to the L1157 bullets (n(H2) ~1-5 10^{5} cm^{-3}, T ~
100-300 K). In both the sources, there is a clear evidence of the presence of
velocity components having different excitation conditions, with the denser
and/or warmer gas associated with the gas at the higher speed. In L1448 the
bulk of the emission is due to the high-excitation and high velocity gas, while
in L1157 most of the emission comes from the low excitation gas at ambient
velocity. The observed velocity-averaged line ratios are well reproduced by
shocks with speeds v_s larger than ~ 30 km/s and densities ~ 10^{5} - 10^{6}
cm^{-3}. Plane-parallel shock models, however, fail to predict all the observed
line profiles and in particular the very similar profiles shown by both low and
high excitation lines. The overall observations support the idea that the L1157
clumps are shock interaction events older than the L1448 bullets close to the
driving source. In the latter objects, the velocity structure and the
variations of physical parameters with the velocity resemble very closely those
found in optical/IR jets near the protostar, suggesting that similar launching
and excitation mechanisms are also at the origin of collimated jets seen at
millimetre wavelengths.
