  Galactic cosmic rays (GCRs) are affected by solar modulation while they
propagate through the heliosphere. The study of the time variation of GCR
spectra observed at Earth can shed light on the underlying physical processes,
specifically diffusion and particle drifts. Recently, the AMS-02 experiment
measured with very high accuracy the time variation of the cosmic ray proton
and helium flux between May 2011 and May 2017 in the rigidity range from 1 to
60 GV. In this work, a comprehensive three-dimensional (3D) steady-state
numerical model is used to solve Parker's transport equation and is used to
reproduce the monthly proton fluxes observed by AMS-02. We find that the
rigidity slope of the perpendicular mean free path above 4 GV remains constant,
while below 4 GV it increases during solar maximum. Assuming the same mean free
paths for helium and protons, the models are able to reproduce the time
behavior of the p/He ratio observed by AMS-02. The dependence of the diffusion
tensor on the particle mass-to-charge ratio, A/Z, is found to be the main cause
of the time dependence of p/He below 3 GV.
