Spiral arms, cosmic rays, and ice ages

Most cosmic rays (CRs) are thought to originate in supernovae, and most supernovae occur in the wake of galactic spiral density waves. Nir Shaviv of the University of Toronto and Jerusalem’s Hebrew University has developed a new CR diffusion model that includes the presence of arms. Not surprisingly, he found that the CR flux at Earth would vary with time and be correlated with our Solar System’s passage through galactic spiral arms as we circumnavigate the Milky Way. He then looked at a historical record of CR flux, in the form of 42 age-dated iron meteorites whose exposures to CRs could be determined. He found a periodicity of 143 million years in the CR flux, which he attributes to passages through spiral arms. On the assumption that CRs ionize Earth’s lower atmosphere and can thus influence climate, Shaviv next looked at the geologic record for ice ages and found “a compelling correlation” of both period and phase between CR flux and glaciation epochs during the past billion years. Between 1 and 2 billion years ago, there is no evidence for any ice age, consistent with a slowed star-formation rate during that period of our galaxy’s history. Shaviv says that the weakest link in his analysis is the uncertainties in the glaciological record. (N. J. Shaviv, Phys. Rev. Lett. 89, 051102, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.051102 .)