Towards Understanding the Time Variations of Proton to Helium Ratios in the Heliosphere: Implication for the Time Dependence of the Elements of the Diffusion Tensor

Abstract

A comprehensive three-dimensional numerical model for the modulation of cosmic rays in the heliosphere is applied to investigate the relative roles of the time dependence of the elements of the diffusion tensor on the proton to total helium (p/He) and helium-3 to helium-4 (3He2/4He2) ratios at rigidities below 3 GV. At these rigidities the ratios have been observed by both PAMELA and AMS detectors to have a significant time variation in response to changing solar activity. We found that the contribution of the time dependence of the perpendicular diffusion in the radial direction of the heliosphere is the dominant cause of this observed time variation, especially in the A < 0 cycle, and not any fundamental difference between the solar modulation of galactic protons and helium isotopes. It follows that neglecting this time dependence, both in value and rigidity dependence, from numerical models would produce time trends in the mentioned ratios that are incompatible with observed trends at the Earth. Furthermore, we found significant differences in the computed time trends of p/He and 3He2/4He2 ratios at rigidities below 1.5 GV. This is mainly a consequence of an interplay between perpendicular diffusion in the radial direction and adiabatic energy losses which begin to influence modulated spectra at a higher rigidity for 3He2 than for 4He2, and for total helium than for protons.