Coronal shocks and solar energetic proton events

From 1996 July through 2001 June, less than half (43/98) of all favorably located (from solar western hemisphere sources) metric type II radio bursts were associated with solar energetic proton (SEP) events observed at Earth. When western hemisphere metric type IIs were accompanied by decametric-hectometric (DH; 1 - 14 MHz) type II emission (observed by Wind/WAVES) during this period, their association with similar to20 MeV SEP events (with peak fluxes greater than or equal to 10(-3) protons cm(-2) s(-1) sr(-1) MeV-1) was 90% (26/29), versus only 25% (17/69) for metric IIs without a DH counterpart. Overall, 82% (63%) of all SEP events with visible disk origins were associated with metric (DH) type II bursts, with the percentage associations increasing with SEP event size to 88% (96%) for similar to20 MeV SEP events with peak intensities of greater than or equal to10(-1) protons cm(-2) s(-1) sr(-1) MeV-1. Our results are consistent with the following possibilities (which are not mutually exclusive): (1) large similar to20 MeV SEP events result from strong shocks that are capable of persisting well beyond similar to3 R-. (the nominal 14 MHz plasma level); (2) shock acceleration is most efficient above similar to3 R-.; and (3) shocks that survive beyond similar to3 R-. are more likely to have broad longitudinal extents, enabling less well connected shocks to intercept open field lines connecting to Earth.