Combined heliospheric modulation of galactic protons and helium nuclei from solar minimum to maximum activity related to observations by PAMELA.

Abstract

The global features of the modulation of galactic cosmic ray protons and helium nuclei are studied in the heliosphere from minimum to maximum solar activity with a comprehensive, three-dimensional drift model and compared to observations measured by PAMELA and AMS02 taken between 2006 and 2017. Combined with accurate very local interstellar spectra (VLIS) for protons and helium nuclei, this provides the opportunity to study in detail how the proton to helium (p/He) ratio at different rigidities behaves with increasing solar activity, through solar maximum conditions and afterward. In particular, the effects at the Earth of the difference in their VLIS’s, mass-to-charge ratio (A/Z) and those caused by the main modulation mechanisms are investigated. We find that the rigidity slopes of the parallel and perpendicular diffusion coefficients below 4 GV should change differently before solar maximum than afterwards to reproduce the observed p/He ratio, in addition to scaling down (up) the values of the diffusion and drift coefficients towards (after) solar maximum. The compatibility between the model computations and observations indicate that the PAMELA and AMS02 measurement of p/He above ~ 5 GV precisely reveals the difference in the rigidity slopes of their VLIS’s at these rigidities.