Boundary Treatment for the Subsonic/Alfvenic Inner Boundary at 2.5 R ? in a Time-dependent 3D Magnetohydrodynamics Solar Wind Simulation Model

A new magnetohydrodynamics ( MHD) simulation model of the global solar corona and solar wind is presented. The model covers the range of heliocentric distance from 2.5 solar radii, so that coronal mass ejections at the earliest phase near the Sun can be treated in the future. This model is constructed by introducing a characteristicsbased boundary treatment to an existing heliosphere 3D MHD model. In tailoring a set of characteristic equations for this new model, we assume that the coronal magnetic field is open to interplanetary space and that the solar coronal plasma is flowing outward everywhere at 2.5 solar radii. The characteristic equations for the subsonic/ Alfvenic inner boundary surface are satisfied by altering the plasma density and/or temperature to maintain a polytropic relationship. In this article, the details of the characteristics-based boundary treatment for the middle of the corona (named CharM) are provided. The quasi-steady states of the solar wind derived from simulations with various choices of a parameter in the boundary treatments are compared and examined. Although further improvements are needed, we apply the new boundary treatment to simulations for three Carrington rotation periods from the minimum to maximum phase of the solar activity cycle, and show that an optimal choice yields a reasonable quasi-steady state of the transonic/Alfvenic solar wind matching the specified subsonic/Alfvenic plasma speed at 2.5 Re.