  We present the X-ray images of all the available Chandra observations of the
galactic jet source SS 433. We have studied the morphology of the X-ray images
and inspected the evolution of the arcsec X-ray jets, recently found to be
manifestations of in situ reheating of the relativistic gas downstream in the
jets. The Chandra images reveal that the arcsec X-ray jets are not steady long
term structures; the structure varies, indicating that the reheating processes
have no preference for a particular precession phase or distance from the
binary core. Three observations made within about five days in May 2001, and a
60 ks observation made in July 2003 show that the variability of the jets can
be very rapid, from timescales of days to (possibly) hours. The three May 2001
images show two resolved knots in the east jet getting brighter one after the
other, suggesting that a common phenomenon might be at the origin of the
sequential reheatings of the knots. We discuss possible scenarios and propose a
model to interpret these brightenings in terms of a propagating shock wave,
revealing a second, faster outflow in the jet.
