THE SHAPES OF GALACTIC COSMIC-RAY INTENSITY MAXIMA AND THE EVOLUTION OF THE HELIOSPHERIC CURRENT SHEET

The observation of a triangular-shaped galactic cosmic ray (GCR) inten sity maximum during the mid-1980s solar minimum was successfully predi cted by drift models of cosmic ray modulation. The assumption underlyi ng this prediction was that the evolution of solar activity, represent ed in drift models by the time development of the ''tilt'' angle of th e heliospheric current sheet (HCS), does not vary greatly from cycle t o cycle. Tilt angles derived from coronal brightness distributions in the 1970s show that this assumption, seemingly supported by the succes sful prediction, may not be valid. The evolution of the HCS during the 1970s, when the 11-year GCR maximum exhibited a broad peak, may have been significantly different (with a broader and less regular period o f low tilt angles) from that inferred for the preceding mid-1960s sola r minimum and from that determined by the Wilcox Solar Observatory dur ing the following mid-1980s minimum. Had the 1970s corresponded to an A < 0 epoch (in the drift formulation), it appears that the resultant GCR intensity maximum would have been double peaked, with a deep rift corresponding to the 1974 minicycle. The different sensitivities of GC R intensity to tilt angle changes in A positive and A negative solar c ycles, as demonstrated by other authors, provide support for drift mod els of modulation. The point we make here is that nonsystematic evolut ion of the HCS from one cycle to the next can be an additional source of variation in the shapes of consecutive GCR intensity maxima.