A Global MHD Simulation of Outer Heliosphere Including Anomalous Cosmic-Rays

Abstract

Abstract A global MHD–neutrals–cosmic-rays simulation is conducted to investigate the effects of anomalous cosmic-rays (ACRs) on the large-scale structure of the outer heliosphere. In the model, the cosmic-rays are treated as a massless fluid that only contribute their pressure to the dynamics of the system. The diffusion of cosmic-rays in the interstellar medium is assumed to be much faster than inside the heliosphere, where it depends on the strength of the magnetic field. The results show that the influence of the cosmic-rays on the structure of the outer heliosphere depends on momentum and energy transfer from the solar wind plasma to the ACRs, accomplished through diffusive shock acceleration at the termination shock, and the subsequent loss of ACRs across the heliopause and their rapid escape into the interstellar medium. Under favorable conditions characterized by a large fraction of energy conversion and a high enough diffusion coefficient in the solar wind, the ACRs were found to reduce the width of the heliosheath by up to ∼18 au. Consequently, these results indicate that the effect of cosmic-rays is a potential key factor for the global structure of the outer heliosphere in a computer model that could partially explain the timing of the heliopause encounters of the two Voyager probes.