Magnetohydrodynamic Simulations of the Solar Corona and Solar Wind Using a Boundary Treatment to Limit Solar Wind Mass Flux

Magnetohydrodynamic simulations of the solar corona and solar wind are sensitive to conditions in the sub-Alfvénic plasma at the base of the solar corona because the structure of the simulated solar corona is determined by the pressure balance of the plasma and the magnetic field. Therefore, it is important to construct an adequate boundary treatment for the sub-Alfvénic surface, and it is highly preferable to build the model from observation-based constraints and basic mathematical theories. We examine new boundary treatments based on the "projected normal characteristic method" for the MHD simulation of the trans-Alfvénic solar wind and solar corona. Our new boundary treatment limits the mass flux escaping through the solar surface. This limit is set to match the solar wind mass flux Ulysses measured during its first fast-latitude scan of the heliosphere. In addition, combining the projected normal characteristic method and the mass flux limit, the model produces reasonable contrasts of the plasma temperature and density between the coronal hole and streamer. A two-dimensional version of the time-dependent MHD simulation with the new boundary treatment is tested using the axisymmetric dipole and quadrupole components of the solar magnetic field observed at solar minimum. The new boundary treatment can be characterized by the specific heat ratio on the surface, and we examined several cases. The solar wind computed with the new boundary treatment matches the Ulysses measurement at r > 1 AU quite well and simultaneously has good contrasts with coronal plasma parameters near the Sun.