Development of a MHD code satisfying solenoidal magnetic field condition and its application to Mercury's magnetosphere

Physics

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2753 Numerical Modeling, 2756 Planetary Magnetospheres (5443, 5737, 6033), 2784 Solar Wind/Magnetosphere Interactions, 6235 Mercury

Scientific paper

The MHD simulation is one of the powerful methods to understand global structure of the magnetosphere. However, in the Mercury's magnetosphere, kinetic effects of plasma might not be negligible because of its small scale. Statistical trajectory tracing of test particles is an important scheme to investigate the kinetic effects of particles. Previous studies by Delcourt et al. [2003; 2005] used analytical models of electric and magnetic fields that are obtained by rescaling the Earth's magnetosphere and calculated the motion of planetary sodium ions. While this approach is efficient to see the dynamics of heavy ions, resultant properties largely depend on the field models. In order to verify the particle dynamics in the more realistic global configuration of the Mercury's magnetosphere, a self-consistent electric and magnetic field configuration such as that obtained from MHD simulations is required. For studies of the kinetic effects, it is important that the resultant magnetic field (B) satisfies solenoidal condition, i.e., divB=0, to avoid artificial acceleration/deceleration.
Aiming at global simulation of the Mercury's magnetosphere, we developed a MHD simulation code that automatically satisfies solenoidal condition for B. To implement the condition, we used vector potential (A) instead of magnetic field itself in the MHD equations. The usage of A automatically guaranteed divB=div(rotA)=0. For an accurate simulation of high Reynolds number magnetofluid, we adopted R-CIP algorithm [Yabe et al., 1991; Xiao et al., 1996] to solve the advection term in the simulation code. The non-advection terms are solved by 4th order Runge-Kutta method or 3rd order Adams-Moulton predictor-corrector method. The code assessment by comparison with previous simulations with TVD algorithm or analytical solutions shows reasonably good ability of energy and mass conservation, and description of MHD discontinuities. A remarkable feature of the new code with A is the precise description of Alfven wave propagation compared to the code with B even for high wave number regime near the Nyquist wavelength. The two-dimensional feature of the code is tested by a simulation of the Kelvin-Helmholtz instability (KHI). The linear growth rate of the fast growing mode agrees well with the linear theory of the KHI and energy conservation is fulfilled reasonably well. In the presentation, initial results of the 3-D global MHD simulation of the Mercury's magnetosphere will be also presented in addition to the basic characteristics of the new code. Reference: Delcourt et al., A quantitative model of the planetary Na+ contribution to Mercury's magnetosphere, Ann. Geophys., 2003. Delcourt et al., Electron dynamics during substorm dipolarization in Mercury's magnetosphere, Ann. Geophys., 2005. Yabe and Aoki, A universal solver for hyperbolic equations by cubic-polynomial interpolation I. One-dimensional solver, Comput. Phys. Commun., 1991. Xiao et al., Constructing oscillation preventing scheme for advection equation by rational function, Comput. Phys. Commun., 1996

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