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The Question

(Submitted April 18, 2006)

Have we been able to detect periodic cycles in the galactic cosmic ray flux (i.e. independent of the solar wind cycles)?

The Answer

Thanks for your excellent question. The most notable periodicity of galactic cosmic rays, is as you stated, due to the solar wind 11 & 22-year cycles. However, scientists do also see periodicity in the beryllium-10 (and carbon-14) content in deep ocean sediments. Beryllium-10 is produced when cosmic rays interact with the Earth's atmosphere and is later stored in deep ocean sediments or in glaciers. The periodic cycles observed in beryllium-10 abundance are associated with sunspot activity which in turn is correlated with glacial cycles. One theory is that the Sun's solar activity varies over longer time scales than just the 11 & 22 year cycles. Another theory postulates that the Earth experiences small changes in its orbit which in turn can affect the Earth's magnetic field and thus the number of cosmic rays entering the Earth's atmosphere. We recommend a very interesting article on the subject:
http://www.sciencedaily.com/releases/2004/08/040803093903.htm

Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"

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