Towards a deeper understanding of historic cosmic ray modulation during solar cycle 20

Abstract

Given recent successes in modelling the transport of cosmic rays (CRs) in the heliosphere from first principles, taking into account the behaviour of heliospheric turbulence quantities and their influence on the transport parameters of these particles in a manner as self-consistent as possible, the question naturally arises as to what insights such a modelling approach can yield when applied to the historical study of cosmic ray transport. The temporal relationship between the heliospheric magnetic field (HMF) and observed CR intensities at Earth during solar cycle 20 has long been regarded as anomalous with respect to other solar cycles. The present study presents an analysis of historic HMF observations that gleans information as to the behaviour of turbulence quantities during this period. This information is used to analyse CR transport coefficients, which are incorporated into a stochastic CR modulation code. Galactic CR intensities so computed are compared with observations, and it is demonstrated that this approach yields a temporal intensity profile in qualitative agreement with that displayed by neutron monitor observations, thereby providing new insights as to CR modulation in solar cycle 20