Research progress and future prospect of radiative magnetohydrodynamics numerical simulations in astrophysics and space physics

Abstract

Over 99% of the baryonic matter in the universe is in the state of plasmas, where matter interacts strongly with the magnetic field. Radiation is generally involved in such interactions, hence affecting the dynamics of the plasmas. All these interactions are described by radiation magnetohydrodynamics (MHD) equations. Because of the strong non- linearity and the multi-scale coupling, only a very limited number of analytical solutions to the MHD equations have been obtained with various approximations and simplifications. To fully understand the physical processes involved in the violent astrophysical eruptions, we have to resort to numerical simulations, which are becoming more and more important in solving the equations and helping us understand the mechanisms hidden in the universe. Owing to the development of the computation hard wares and numerical algorithms, numerical simulations, together with theoretical analyses and experiments (or observations), become three major methods in nowadays research. In particular, numerical simulations have made tremendous progress in the past decades. This paper reviews the research progress of radiation MHD numerical simulations made in astrophysics (including solar physics) and space science during the past 10 years. Their future prospect is also envisioned. Limited by space, only the following topics are included in this review paper. (1) Radiation MHD numerical simulation has become more and more important in various aspects of astrophysical research. In this review, we take black hole accretion as an example. This is mainly because radiation MHD simulation is best developed and applied in this field. In fact, almost all the important progresses in the past 20 years in this field were achieved by radiation MHD numerical simulations. In this review, we introduce three examples. One is the magneto-rotational instability, which is widely believed to be the mechanism of angular momentum transport in accretion flows. The second one is the wind launched from black hole accretion flows, including why we believe the existence of strong winds and what the mechanisms of wind production are. The third topic is the super-Eddington accretion. We introduce how radiation MHD numerical simulations have completely changed our traditional paradigm that was reached by analytical studies. (2) In respect of solar physics, after briefly mentioning the necessity of radiation MHD numerical simulations in the solar atmosphere research, we introduce four different approaches in dealing with the radiation process based on different parameter regimes of various atmospheric layers, such as the photosphere, chromosphere, transition region, and the corona. We further describe the numerical results related to several crucial topics in solar physics, including the formation of several key spectral lines, the structure of the lower solar atmosphere, magnetic reconnection in the lower solar atmosphere, chromospheric and coronal heating, formation of coronal loops, filament formation, and so on. (3) As an illustrative example of typical applications of RMHD in space physics, we offer an overview as to how RMHD is incorporated into the numerical simulations of the solar wind in the inner heliosphere. For this purpose, we first offer a brief introduction to the consensus on the origins of the solar wind, thereby showing that the acceleration of the nascent solar wind is very likely to be a direct result of coronal heating in magnetically open regions. We go on to show that radiation needs to be carefully addressed to self-consistently account for the wind thermodynamics. With some illustrative examples we show how radiation is handled in current multi-dimensional solar wind models.