Major Solar Energetic Particle Events of Solar Cycles 22 and 23: Intensities Close to the Streaming Limit

It has been argued that the highest intensities measured near 1 AU during large solar energetic particle events occur in association with the passage of interplanetary shocks driven by coronal mass ejections, whereas the intensities measured early in the events (known as the prompt component) are bounded by a maximum intensity plateau known as the streaming limit. A few events in Solar Cycle 23 showed prompt components with intensities above the previously determined streaming limit. One of the scenarios proposed to explain intensities that exceed this limit in these events invokes the existence of transient plasma structures beyond 1 AU able to confine and/or mirror energetic particles. We study whether other particle events with prompt-component intensities close to the previously determined streaming limit are similarly affected by the presence of interplanetary structures. Whereas such structures were observed in four out of the nine events studied here, we conclude that only the events on 22 October 1989, 29 October 2003, and 17 January 2005 show interplanetary structures that can have modified the transport conditions in a way similar to those events with prompt components exceeding the previously determined streaming limit. The other six events with prompt components close to the previously determined streaming limit were characterized by either a low level of pre-event solar activity and/or the absence of transient interplanetary structures able to modify the transport of energetic particles.