| abstract: |
Measurements of cosmic-ray-induced nuclides such as carbon-14 and beryllium-10 in tree rings and ice cores have revealed that length of the 11-year solar activity cycle changes in time associated with the centennial-scale variation of solar activity level. For example, solar cycles are lengthened to be about 14 years around the period of long-term weakening of solar activity such as the Maunder Minimum in the 17th century. Accordingly, the “22-year” period of solar magnetic polarity reversals also changes in time. Detailed analyses of beryllium-10 data from Greenland ice core have revealed that the pattern of the “22-year” variation in the incident galactic cosmic rays also changes according to the long-term solar activity level. This can be explained by long-term variation of large-scale structure of heliospheric magnetic field, which is playing important role in modulating the galactic cosmic rays. The reversal of solar dipole magnetic polarity alters the trajectory of cosmic rays in the heliosphere, and hence results in the 22-year component in the cosmic-ray time profile. Comparison between the reconstructed cosmic-ray flux and northern hemispheric climate during the Maunder Minimum has suggested that the variation of heliospheric magnetic structure is playing important role in determining the pattern of multi-decadal climate variations through the modulation of cosmic rays. Cosmic rays are suggested to have an influence on cloud formation or on the life time of clouds, although the detailed mechanism is yet to be clarified. In this seminar, we also present the possible pathway of cosmic-ray influence on climate system.
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