Characteristics of the 22‐year modulation of cosmic rays as seen by neutron monitors

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Abstract
Using neutron monitor data from 1952 through the end of 1987, including the maximum intensity in early 1987, we have observed several new features which illustrate the importance of a 22‐year modulation cycle in addition to the well‐known 11‐year cycle. These features include a sharply peaked intensity maximum in 1987, similar to that observed 22 years earlier in 1965, in contrast to the flatter maximum observed between 1972 and 1977 and earlier in 1952–1954. We also find that the neutron monitor intensity is ∼1.5% higher at the time of the 1987 maximum and also in 1965 as compared to the maximum in the 1972–1977 period. Systematic differences in the overall shape of successive 11‐year modulation cycles and similarities in the shape of alternate 11‐year cycles are observed that seem to be related to the 22‐year magnetic cycle, to predictions of drift models, and to the effects of the equatorial current sheet. A very high correlation is observed between the tilt of this current sheet and the neutron monitor intensity over the entire 1984–1987 period. An abrupt increase in the current sheet tilt that occurs in May–June 1987 seems to be directly related to the intensity decreases observed by neutron monitors in the latter part of 1987.