Ken McCracken
IPST, University of Maryland
Cosmic Ray Modulation Over the Past 10,000 Years
The cosmogenic nuclides, 10Be and 14C, provide two independent records of the cosmic ray intensity at Earth, each stretching back >10,000 years. They exhibit several prominent and large amplitude periodicities; the Hale (11/ 22yr); Gleissberg (~85yr): de Vries (208 yr); and Hallstatt (2200 yr). In the period under study, there have been twenty two “Grand Minima” events in the cosmic ray intensity, similar to those accompanying the Maunder (1645-1715) and Spoerer (1420-1540) minima. During these events, the cosmic ray intensity at Earth has approached that of the local interstellar spectrum. These features of the paleo-cosmic ray record will be discussed, showing that the “instrumental era” has represented one of the more active intervals (but not most active) in the past 10,000 yr. The until recently poorly known Hallstatt (2200yr) periodicity will be discussed in some detail, showing that the “Grand Minima” events usually coincide with the Hallstatt minima, leading to the prediction that there may be no more deep Grand Minima for another 1000 years. The cosmogenic data have been used to estimate the strength of the interplanetary magnetic field throughout the 10,000 yr interval, yielding 25 yr averages of ~1 nT during deep Grand Minima, and ~8nT during the maxima of the Hallstatt cycle. The solar implications will be discussed. Finally, the understanding gained from the paleo-cosmic ray record will be used to discuss the significance of the current “prolonged sunspot minimum”, and the potential that it presents to provide improved understanding of the heliospheric conditions in the past.