Testing diffusion of cosmic rays in the heliosphere with proton and helium data from AMS

Abstract

After six years of continuous observations in space, the Alpha Magnetic Spectrometer experiment has released new data on the temporal evolution of the proton and helium fluxes in cosmic rays. These data revealed that the ratio between proton and helium fluxes at the same value of rigidity $R=p/Z$ (momentum/charge ratio) is not constant at $R\lesssim$ 3 GV. In particular, the ratio is found to decrease steadily during the descending phase of Solar Cycle 24 toward the next minimum. We show that such a behavior is a remarkable signature of the $\beta\times\lambda(R)$ dependence in the diffusion of cosmic rays in heliosphere, where $\beta$ is their adimensional speed and $\lambda(R)$ is their mean free path, a universal function of rigidity for all nuclei. This dependence is responsible for distinctive charge/mass dependent effects in the time-dependent modulation of low-rigidity particles.