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

Identification of Super- and Sub-critical Regions in Shocks driven by Coronal Mass Ejections