SUPEREVENTS AND COSMIC-RAY MODULATION, 1974-1985

Superevents are long-lived (approximately 40 days at 1 AU) approximate ly 10-MeV proton events that originate in episodes of intense solar ac tivity characterized by major coronal mass ejections (CMEs) and indivi dual solar energetic particle (SEP) events. Superevents exhibit only w eak intensity variation with heliolongitude. They propagate to the out er heliosphere at speeds above that of the average solar wind, and, at Pioneers 10 and 11, prominent superevents are generally associated wi th strong interplanetary shocks. For the period from 1974 to 1985, we find that superevents are not reliable signatures of the onsets of lon g-term steps in the modulation record of >1-GV galactic cosmic rays (G CRs) at 1 AU. Of six intense superevents during this interval, one occ urred coincident with the onset of a step (June-July 1982), two occurr ed midway through step decreases, and three occurred at the ends of st ep decreases. Two step decreases during this period began in conjuncti on with relatively weak SEP activity. Thus the largest superevents occ urring from 1974 to 1985 were neither necessary nor sufficient conditi ons for long-term GCR intensity steps and therefore were not closely r elated to the global merged interaction regions that have been identif ied 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, CM Es is more important for establishing the 11-year cycle than are the s poradic, and relatively short-lived, outbreaks of major CME activity t hat give rise to superevents.