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Superevents and Cosmic Ray Modulation, 1974-1985
PHILLIPS LAB HANSCOM AFB MA
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Superevents are long-lived proton events that originate in episodes of intense solar activity characterized by major coronal mass ejection CMEs and individual solar energetic particle 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. 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 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. 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 convectiondiffusion 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 occuring, and less energetic,
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