Pre-existing turbulence and large gradual SEP events

Gradual Solar Energetic Particle events (SEPs) are now believed to be associated with shocks driven by Coronal Mass Ejections (CMEs). As CMEs propagate out from the Sun, particles are accelerated at the shock front, reaching energies to 10's of MeV and occasionally ~ GeV's. However, the efficiency and ability of accelerating particles to very high energies vary drastically from event to event. Individual events, whose solar progenitors are similar, can differ much at high energies. Recently, there are observational indications that large gradual events usually happen when two or more CMEs occur closely. In this work, we study the consequence of having two CMEs occur closely in time. We find that the first CME can effectively enhance the turbulence upstream the second shock from its interplanetary level. Such an increase will significantly decrease the acceleration time scale at the second shock and thus boost particles to much higher energies. We show that under certain situation, an increase of 30 for the maximum particle energy can be achieved. We discuss possible implications of our calculation to observations of large gradual SEP events such as the 2005 January 20 event.