Hiroko Miyahara

Institute for Cosmic Ray Research, The University of Tokyo

Comparison between reconstructed solar and cosmic ray variations and the variations of northern hemisphere climate during the Little Ice Age has suggested that solar activity, including the reversals of the dipole magnetic field and the changes in large-scale structure of heliosphere, is playing an important role in determining the pattern of climate change at decadal to multi-decadal time scales through modulation of cosmic rays. These features were revealed by focusing on the Maunder Minimum, when cosmic rays had peculiar ”22-year” variations with much larger amplitudes than today.  The 22-year variation in incident galactic cosmic rays is caused by the polarity reversal of solar magnetic field that determines the convection of cosmic rays in the heliosphere.  Therefore the characteristics of the 22-year variation changes in time, associated with the long-term change in solar activity and the large-scale structure of heliospheric magnetic field. During the Maunder Minimum, when the actual length of the 22-year cycles had been stretched to about 28 years, the pattern of the "22-year" cycle turned out to be unique, probably associated with the more flattened heliospheric current sheet during the time.  It has caused 30-50% increase of incident cosmic rays, lasting only ~1 year, at every other minima of the 14-year cycles. We suggest that such ~1-year peaks can be utilized to trace the impact of cosmic rays on the climate system. Our reconstruction of relative humidity for the rainy season in Japan has suggested that cosmic ray peaks had impacted on monsoonal activity in Asia. To examine the detailed mechanisms, we have also analyzed the record of Outgoing Long-wave Radiation for the past 30 years. Cosmic rays, as well as solar irradiance, have 27-day variations due to solar rotations. We have found that 27-day variations are found in cloud-height change around the tropical area only at the 11-year solar maxima.

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