A Numerical Study of Cosmic Proton Modulation Using AMS-02 Observations

Abstract

Since 2011 May, the Alpha Magnetic Spectrometer (AMS-02) on board the International Space Station has provided monthly cosmic proton fluxes for various low-rigidity levels ( P  < 50 GV). These precise measurements, in terms of high time and rigidity resolution, have provided a good opportunity to study cosmic ray modulation over a wide range of rigidities, together with transient events. Subsequently, a comprehensive numerical transport model has been constructed, based on Parker’s transport equation that includes all known physical mechanisms: diffusion, convection, drift, and adiabatic cooling. Propagating diffusion barriers to simulate Forbush decreases (Fds) and global merged interaction regions (GMIRs) have also been incorporated: (1) utilizing a time-varying tilt angle of the heliospheric current sheet and interplanetary magnetic field, the general trend of the time variation of cosmic proton fluxes has been reproduced; (2) the Fd events in 2011 October and 2012 March have been simulated, and the first GMIR event in solar cycle 24 has also been simulated and studied; and (3) the rigidity dependence of the proton fluxes, as revealed by the AMS-02 data, has been reproduced with the appropriate chosen rigidity dependent diffusion coefficients. In order to reproduce the proton observations, we find that apart from the transient events, the derived mean free paths in interplanetary space also need to be changed with time.