An ab initio approach to the historical modulation of cosmic rays

Abstract

The relationship between observed cosmic-ray intensities and the heliospheric magnetic field during the early space age is generally considered to be anomalous, relative to their relationship observed at later times. Various explanations for this behaviour have been proposed, but these do not take into account the influence of magnetic turbulence on the transport of cosmic-rays. The aim of this study is to take this into account, by analysing historic heliospheric magnetic observations in an appropriatemanner to gain insight as to the behaviour of magnetic variances and correlation lengthscales during this period. These quantities are then used as inputs for an ab initio cosmic-ray modulation model, using theoretically-motivated expressions for cosmic ray diffusion and drift coefficients. Galactic cosmic-ray intensities computed with this model, when compared with neutron monitor observations, are found to be in qualitative agreement. From this it can be concluded that cosmic-ray modulation conditions during the early space age were not as anomalous as previously thought, and that it is essential to take turbulence into account in the study of historic cosmic-raymodulation.