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Astron. Astrophys. 330, 764-772 (1998)

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1. Introduction

The 11-year modulation of cosmic-rays, in anticorrelation with the 11-year solar activity cycle, is well established, although its origin is not fully understood (Forbush, 1958). In addition to this 11-year cycle, there are various features which indicate that a 22-year periodicity in this modulation is also important (Webber and Lockwood, 1988). Among the more important features that seem to have a 22-year periodicity (in addition to an 11-year periodicity) are: the diurnal variation (Thambyahpillai and Elliot, 1953), general asymmetries in the rigidity dependence of the modulation that are different in successive 11-year cycles (Webber et al, 1983), modulation effects that depend on the sign of the particle change and are different in successive 11-year cycles (Garcia-Munoz et al, 1986) etc. Neutron monitor data available now for more than two complete 22-year cycles show evidence for the existence of 22-year periodicity in the cosmic-ray intensity variations (Nagashima and Morishita, 1980b; Mavromichalaki et al, 1988).

It has been pointed out by many researchers that some anomalous phenomena in the solar modulation of cosmic rays have been observed for several years after the solar maximum (1968) in the 20th solar cycle. These phenomena are the abnormality of the modulation rigidity spectra of cosmic-ray intensities (Lockwood and Webber, 1979), the sudden recovery of the intensity (Kuzmin et al., 1977), the poor correlation of the intensities with solar activities (Ashirof et al., 1977) etc. These phenomena were summarised by Nagashima (1977) and interpreted as the result of reversal of the polar magnetic field of the Sun that occurred over the period of mid 1969-mid 1971. Such phenomena in the cosmic-ray intensity have also been observed after solar maximum as well as in the declining phases of the 19th, 21st and 22nd solar cycles (Webber and Lockwood, 1988; Mavromichalaki et al., 1988). Jokipii et al. (1977) suggest that the modulation of the galactic cosmic rays should have a significant component controlled by the state of the interplanetary magnetic field as transported out from the Sun, and hence there should be a solar cycle effect on the drift of cosmic rays in the heliosphere.

The purpose of this contribution is to consider at first some different aspects of the solar modulation during the period 1947-1995 covering five complete solar cycles (18th, 19th, 20th, 21st, 22nd) and to suggest tentative reasons for their different behaviour in different cycles. Examining cosmic-ray monthly values of the Climax and Inuvik Neutron Monitor Stations for the period Jan. 1953 to Dec. 1994, similarities have been found between modulation phenomena of galactic cosmic rays during solar activity cycles of the same type (even or odd). This fact supports a possible effectiveness of the Hale cycle (about a 22-year variation connected with the heliomagnetic field). Obtaining useful information from this consideration, we have simulated the long-term modulation of galactic cosmic rays during the last three cycles, where data are available, according to the method by Mavromichalaki et al. (1990). The computed values follow the observations fairly well, and the observed deviations in cosmic-ray residuals have appeared during or after reversals of the polar magnetic field of the Sun. According to these results, one should expect a 22-year variation in cosmic-ray intensity.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998