Components of the 11- and 22-year variation of cosmic rays

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Long-term cosmic ray observations by means of neutron monitors now allow us to distinguish modulation effects related to the 22-year solar magnetic cycle from effects due to the 11-year solar activity cycle. New results based on the data from two independent pairs of neutron monitor stations with widely differing geomagnetic cutoffs, Climax-Huancayo and Kiel-Tsumeb, are presented. First, a recently reported [Popielawska and Simpson, 1990, in Proc. 1 st COSPAR Colloquium “Physics of the Outer Heliosphere” (Edited by S. Grzedzielski and D. E. Page), Pergamon, Oxford] finding from Climax-Huancayo correlations—that during the ascending phase of the 11-year solar activity cycle the rigidity dependence of modulation depends on the sign of dominant polarity of the heliospheric magnetic field—is confirmed here by using a Kiel-Tsumeb pair of neutron monitors. Secondly, when data for the descending phase of solar activity cycle are analyzed it is found that the rigidity dependence of modulation is apparently the same for A > 0 as for A < 0 polarity. This emerging puzzle is related to other and more fundamental differences in the characteristics of cosmic ray modulation during the A > 0 and A < 0 phases of the solar magnetic cycle. On the one hand, during A < 0 periods the 11-year variation has two components distinguished by the shape of the correlation curve (representing the amplitude of modulation at high vs low rigidities)—rectilinear or oval. The rectilinear shape is primarily observed at low modulation levels, while the oval shape with (closed) hysteresis loops is seen at higher solar activity after strong heliospheric disturbances. On the other hand, the data for periods with A > 0 polarity suggest that the cosmic ray modulation is accompanied by the formation of hysteresis loops covering the whole constant-polarity period centered on sunspot minimum. The reported difference in the rigidity dependence of modulation at A > 0 and A < 0 is predicted by a steady-state drift-dominated modulation model (Reinecke and Potgieter, 1990, Proc. 21 st Int. Cosmic Rays Conf., 6, 17). When the observed phenomenon of spectral hysteresis is interpreted in the frame of a time-dependent model then the second reported result could mean that for A > 0 the modulation at > 1 GV rigidities is accompanied by a significant rigidity-dependent phase lag from the very beginning of the solar activity cycle while for A < 0 this phase lag manifests itself only after strong heliospheric disturbances. The cause of such a difference in cosmic ray modulation during the A > 0 and A < 0 portions of the solar magnetic cycle so far is unclear.