Solar Energetic Particle Production by Coronal Mass Ejection-driven Shocks in Solar Fast-Wind Regions
Gradual solar energetic particle (SEP) events at 1 AU are produced by coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Fast (vCME>~900 km s-1) CMEs might produce stronger shocks in solar slow-wind regions, where the flow and fast-mode MHD wave speeds are low, than in fast-wind regions, where those speeds are much higher. At 1 AU the O+7/O+6 ratios distinguish between those two kinds of wind streams. We use the 20 MeV proton event intensities from the EPACT instrument on Wind, the associated CMEs observed with the LASCO coronagraph on SOHO, and the ACE SWICS solar wind values of O+7/O+6 to look for variations of peak SEP intensities as a function of O+7/O+6. No significant dependence of the SEP intensities on O+7/O+6 is found for either poorly connected or well-connected CME source regions or for different CME speed ranges. However, in the 3 yr study period we find only five cases of SEP events in fast wind, defined by regions of O+7/O+6<0.15. We suggest that in coronal holes SEP acceleration may take place only in the plume regions, where the flow and Alfvén speeds are low. A broad range of angular widths are associated with fast (vCME>=900 km s-1) CMEs, but we find that no fast CMEs with widths less than 60° are associated with SEP events. On the other hand, nearly all fast halo CMEs are associated with SEP events. Thus, the CME widths are more important in SEP production than previously thought, but the speed of the solar wind source regions in which SEPs are produced may not be a factor.