A numerical study of unusual flux decreases for cosmic ray protons and electrons observed by Alpha Magnetic Spectrometer in 2017

Abstract

Aims. Alpha Magnetic Spectrometer (AMS), installed on the International Space Station, delivers precision measurements of cosmic proton fluxes and electron fluxes, providing unique inputs to further improve our understanding of the solar modulation of cosmic protons and electrons. The latest measurements published by AMS show significant decreases in daily cosmic proton fluxes and electron fluxes in the second half of 2017 (approximately from June 11, 2017 to December 23, 2017). A special structure, known as a loop, appears in the electron-proton hysteresis during this period. These declining fluxes, as well as their recovery toward solar minimum modulation, could be attributed to solar wind structures such as global merged interaction regions (GMIRs), which can affect cosmic ray flux for several months, as well as coronal mass ejections (CMEs). We aim to find the reason for the decrease and clarify the solar modulation mechanism underlying the loop structure.Methods. We developed a 3D numerical model based on Parker transport equation, which is solved as a set of stochastic differential equations, combined with diffusion barriers propagating away from the Sun. Correspondingly, the relevant parameters can be tuned up.Results. The unusual changes in cosmic proton fluxes and electron fluxes in the second half of 2017 could be caused by CMEs and GMIRs. The decreases in these fluxes in 2017, with rigidities below 11 GV, have been successfully reproduced. Daily variations at Earth in terms of the diffusion coefficients (and their mean-free paths) were subsequently obtained. Furthermore, our simulation reveals that the electron-proton hysteresis loop structure in 2017 results from the different responses of protons and electrons to solar modulation, especially with respect to drift and diffusion processes in the heliosphere.