  We study solar energetic particle (SEP) events during multiple solar
eruptions. The analysed sequences, on 24-26 November 2000, 9-13 April 2001, and
22-25 August 2005, consisted of halo-type coronal mass ejections (CMEs) that
originated from the same active region and were associated with intense flares,
EUV waves, and interplanetary (IP) radio type II and type III bursts. The first
two solar events in each of these sequences showed SEP enhancements near Earth,
but the third in the row did not. We observed that in these latter events the
type III radio bursts were stopped at much higher frequencies than in the
earlier events, indicating that the bursts did not reach the typical plasma
density levels near Earth. To explain the missing third SEP event in each
sequence, we suggest that the earlier-launched CMEs and the CME-driven shocks
either reduced the seed particle population and thus led to inefficient
particle acceleration, or that the earlier-launched CMEs and shocks changed the
propagation paths or prevented the propagation of both the electron beams and
SEPs, so that they did not get detected near Earth even when the shock arrivals
were recorded.
