Ew. Cliver, THE SHAPES OF GALACTIC COSMIC-RAY INTENSITY MAXIMA AND THE EVOLUTION OF THE HELIOSPHERIC CURRENT SHEET, J GEO R-S P, 98(A10), 1993, pp. 17435-17442
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.