SUPEREVENTS AND COSMIC-RAY MODULATION, 1974-1985

Superevents are long-lived (approximately 40 days at 1 AU) approximately 10-MeV proton events that originate in episodes of intense solar activity characterized by major coronal mass ejections (CMEs) and individual solar energetic particle (SEP) events. Superevents exhibit only weak intensity variation with heliolongitude. They propagate to the outer heliosphere at speeds above that of the average solar wind, and, at Pioneers 10 and 11, prominent superevents are generally associated with strong interplanetary shocks. For the period from 1974 to 1985, we find that superevents are not reliable signatures of the onsets of long-term steps in the modulation record of >1-GV galactic cosmic rays (GCRs) at 1 AU. Of six intense superevents during this interval, one occurred coincident with the onset of a step (June-July 1982), two occurred midway through step decreases, and three occurred at the ends of step decreases. Two step decreases during this period began in conjunction with relatively weak SEP activity. Thus the largest superevents occurring from 1974 to 1985 were neither necessary nor sufficient conditions for long-term GCR intensity steps and therefore were not closely related to the global merged interaction regions that have been identified with such steps. In terms of convection/diffusion models of cosmic ray modulation, the poor association of the largest superevents with long-term GCR intensity decreases during this period suggests that the background level of more frequently occurring, and less energetic, CMEs is more important for establishing the 11-year cycle than are the sporadic, and relatively short-lived, outbreaks of major CME activity that give rise to superevents.