Simulations of the solar modulation of Helium isotopes constrained by observations

Abstract

The observation of galactic cosmic ray (GCR) Helium isotopes (3He2 and 4He2) at the Earth had been done precisely with the PAMELA and AMS02 space detectors and reported from July 2006 to December 2007 and from May 2011 to May 2017, respectively. These observations span time frames that include solar maximum activity and the magnetic field reversal epoch. A comprehensive, three-dimensional numerical modulation model for the transport of GCRs in the heliosphere is utilized to simulate the modulation of galactic Helium isotopes from the end of the minimum activity in the previous A < 0 cycle, through solar maximum, and toward minimum activity in the current A > 0 cycle. A particular objective is to reproduce the main features of the 3He2 to 4He2 ratio observed at rigidities between 2.15 GV and 15.3 GV. We find that the model reproduces the apparent single power law rigidity of this ratio and its time dependence in good agreement with observations. The simulated results indicate that in addition to the significant change of particle drift with solar activity, the rigidity dependence of the diffusion coefficients below 4 GV change differently before solar maximum activity than afterwards. This influences the unfolding of the modulation of GCRs with changing solar activity differently when they have different very local interstellar spectra and mass-to-charge ratios.