  The detection of narrow SiO thermal emission toward young outflows has been
proposed to be a signature of the magnetic precursor of C-shocks. Recent
modeling of the SiO emission across C-shocks predicts variations in the SiO
line intensity and line shape at the precursor and intermediate-velocity
regimes in only few years. We present high-angular resolution (3.8"x3.3")
images of the thermal SiO J=2-1 emission toward the L1448-mm outflow in two
epochs (November 2004-February 2005, March-April 2009). Several SiO
condensations have appeared at intermediate velocities (20-50 km/s) toward the
red-shifted lobe of the outflow since 2005. Toward one of the condensations
(clump D), systematic differences of the dirty beams between 2005 and 2009
could be responsible for the SiO variability. At higher velocities (50-80
km/s), SiO could also have experienced changes in its intensity. We propose
that the SiO variability toward L1448-mm is due to a real SiO enhancement by
young C-shocks at the internal working surface between the jet and the ambient
gas. For the precursor regime (5.2-9.2 km/s), several narrow and faint SiO
components are detected. Narrow SiO tends to be compact, transient and shows
elongated (bow-shock) morphologies perpendicular to the jet. We speculate that
these features are associated with the precursor of C-shocks appearing at the
interface of the new SiO components seen at intermediate velocities.
