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

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6. Conclusions

We can summarise the following:

1.- A distinction between even and odd solar cycles, as well as between the declining and ascending phases of them, is well established. As concerns the solar activity there are symmetrical and asymmetrical solar cycles where generally the rise is faster and the decline is longer. As concerns the cosmic-ray modulation during solar cycles, the shapes of the cosmic-ray curves of the even cycles differ systematically and markedly from the shape of the odd cycles. The odd cycle is characterised by a simple and relatively smooth increase to the maximum (7.5 yr), whereas the even cycles on the average are characterised by two maxima. The first maximum is reached relatively rapid, after the previous minimum in cosmic-ray intensity (3-4 yr). The second, the main and also more developed, tends to occur at the same time in the cycle as the maximum of the odd cycle.

2.- A proposed model described the long term modulation of cosmic rays over three solar cycles closely relates the long to the short term cosmic-ray modulation, and it is able to explain the long-term modulation overcoming the difficulties that arise when the interplanetary parameters are assumed almost constant in the quasi-stationary convection-diffusion model. The results obtained are very satisfactory through the whole period under study, which covers three solar-cycles. It is assumed that this model perhaps will give an integrated model for the cosmic-ray modulation for the coming solar cycles.

3.- The hysteresis loops obtained from the cosmic-ray data of Inuvik, as well as those obtained from Eq. (1) present the expected differences between even and odd cycles. The proposed model works very well through the three solar cycles, denoting the transition from the parallel to the antiparallel state of the solar magnetic field and vice versa in the hysteresis curves.

All these solar cycle phenomena occurring during even and odd solar cycles give evidence for the existence of a 22-year variation in cosmic-ray intensity. This interpretation is based on a working hypothesis that when the polar magnetic field of the Sun is nearly parallel to the galactic magnetic field, they could easily connect, so that galactic cosmic-rays, especially those of low rigidities, could intrude more easily into the heliomagnetosphere along the magnetic lines of force, as compared with those in the antiparallel state of the magnetic fields. Different processes then influence cosmic-ray transport in the heliosphere. During even cycles, convection plays the most important role, while diffusion dominates during odd cycles. In more recent times, the effect of gradient drifts in the oppositely-directed north and south heliospheric magnetic fields has received much attention (Mckibben, 1990; Potgieter, 1994). The drift picture of Kota and Jokipii (1983) fits naturally into the 22-year periodicity of the solar magnetic field. A further study of this model with more suitable source functions that can be associated with the electromagnetic properties in the modulating region will lead us to a better understanding of the relations among coronal structure, interplanetary structure and cosmic rays.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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