  In this work, we focus on the analysis of a CME-driven shock observed by
SOHO/LASCO. We show that white-light coronagraphic images can be employed to
estimate the compression ratio X = rho_d / rho_u all along the front of
CME-driven shocks. X increases from the shock flanks (where X ~ 1.2) to the
shock center (where X ~ 3.0 is maximum). From the estimated X values, we infer
the Alfv\'en Mach number for the general case of an oblique shock. It turns out
that only a small region around the shock center is supercritical at earlier
times, while higher up in the corona the whole shock becomes subcritical. This
suggests that CME-driven shocks could be efficient particle accelerators at the
initiation phases of the event, while at later times they progressively loose
energy, also losing their capability to accelerate high energy particles. This
result has important implications on the localization of particle acceleration
sites and in the context of predictive space weather studies.
