Super- and sub-critical regions in shocks driven by radio-loud and radio-quiet CMEs

White-light coronagraphic images of coronal mass ejections (cmes) observed bysoho/lasco c2 have been used to estimate the density jump along the whole front of twocme-driven shocks. the two events are different in that the first one was a ‘‘radio-loud’’ fast cme, while the second one was a ‘‘radio quiet’’ slow cme. from the compression ratios inferred along the shock fronts, we estimated the alfve´n mach numbers for the general case of an oblique shock. it turns out that the ‘‘radio-loud’’ cme shock is initially super-critical around the shock center, while later on the whole shock becomes sub-critical. on the contrary, the shock associated with the ‘‘radio-quiet’’ cme is sub-critical at all times. this suggests that cme-driven shocks could be efficient particle accelerators at the shock nose only at the initiation phases of the event, if and when the shock is super-critical, while at later times they lose their energy and the capability to accelerate high energetic particles.