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Divergence-free Approximate Riemann Solver for the Quasi-neutral Two-fluid Plasma Model

A numerical method for the quasi-neutral two-fluid (QNTF) plasma model is described. The basic equations are ion and electron fluid equations and the Maxwell equations without displacement current. The neglect of displacement current is consistent with the assumption of charge neutrality. It thus reduces to the ideal magnetohydrodynamic (MHD) equations in the long wavelength limit, but the two-fluid effect appearing at ion and electron inertial scales is fully taken into account. It is shown that the basic equations may be rewritten in a form that has formally the same structure as the MHD equations. The total mass, momentum, and energy are all written in the conservative form. A new three-dimensional numerical simulation code has been developed for the QNTF equations. The HLL (Harten-Lax-van Leer) approximate Riemann solver combined with the upwind constrained transport (UCT) scheme is applied. The method was originally developed for MHD (Londrillo & Del Zanna, 2004), but works quite well for the present model as well. The simulation code is able to capture sharp multidimensional discontinuities as well as dispersive waves arising from the two-fluid effect at small scales without producing $\nabla \cdot \mathbf{B}$ errors. It is well known that conventional Hall-MHD codes often suffer a numerical stability issue associated with short wavelength whistler waves. On the other hand, since finite electron inertia introduces an upper bound to the phase speed of whistler waves in the present model, our code is free from the issue even without explicit dissipation terms or implicit time integration. Numerical experiments have confirmed that there is no need to resolve characteristic time scales such as plasma frequency or cyclotron frequency for numerical stability. Consequently, the QNTF model offers a better alternative to the Hall-MHD or fully electromagnetic two-fluid models.