Record of the decadal and centennial solar cycles in meteorites

Cosmogenic radionuclides, produced in meteorites from nuclear interactions of galactic cosmic rays with meteoroids in the interplanetary space, record the effects of the cosmic ray modulaton induced by solar activity variations. Measuring different radionuclides with different half-lives it is possible to investigate over different time scales. Measurement of the 22Na (T1/2 = 2.6 yr)/26Al(T1/2) = 7.3×10^^5yr) ratio in chondrites with different time of fall in the last decades shows clearly the record of the 11 yr solar cycle. Both phase and amplitude variations agree with those expected. Measurement of the very low activity of the 44Ti (T1/2 = 29.2 yr), normalized to iron and nickel element abundance, in meteorites which fell in the past 150 years show clearly a centennial scale modulation. The phase of the 44Ti profile agrees with that expected, but the amplitude of the variations is 3.5-4 times higher than calculated adopting a cosmic ray flux in the past deduced from the available measurements in the last few decades and extrapolated up to 1750 solely from the sunspot number series. We deduce that during prolonged solar activity minima, such as the Dalton and the Modern minima the heliospheric magnetic field was weaker and admitted a higher cosmic ray flux respect to that observed in the recent minima of the 11 yr solar cycles characterized by broad variations of the sunspot number.