SM53C-1409
The Plasma Environment Of Venus: Comparison Of Venus Express Aspera-4 Measurements With 3D Hybrid Simulations
We use data of the ASPERA-4 ion and electron spectrometers onboard Venus Express (VEX) to determine the locations and shapes of the plasma boundaries (bow shock, ion composition boundary and mantle) at Venus and compare our fits with previous models obtained by PVO and Venera 9, 10 observations. We also investigate the variation of the terminator bow shock position as a function of the solar wind dynamic pressure and solar EUV flux. We compare the results with a 3D hybrid simulation, originally developed for the interaction of the solar wind with weak comets. In the hybrid model, ions are treated as particles moving in self-consistently generated electromagnetic fields and electrons are modeled as a massless charge neutralizing fluid. The planetary heavy ion plasma is generated by an oxygen ionosphere and exosphere adapted to a profile, which depends on the solar zenith angle (Chapman layer). The hot oxygen exosphere is modeled based on the electron density profile obtained from the VeRa experiment onboard VEX. The comparison of the model data with the boundary positions and plasma moments obtained by the ASPERA-4 experiment allows us evaluate the accuracy of the model which then can be used to estimate boundary positions and escape fluxes from the planet under conditions different from today.
SM53C-1410
Thermalization of Na+ Pickup Ions in Mercury's Magnetosheath and Magnetosphere via Hybrid Simulation
In previous studies it has been suggested that the incorporation of Na+ pickup ions into Mercury's magnetosphere could have a significant impact on various magnetospheric processes. Test particle simulations indicate that freshly created Na+ ions are rapidly energized and lost from the system. In order to incorporate these ions into the bulk magnetospheric plasma they must be thermalized. A recent study that used linear theory suggests that the wavelengths of electromagnetic ion cyclotron waves may be to large and may not grow to sufficient amplitudes to thermalize these ions and concluded that global thermalization of these ions is not possible. However, under certain solar wind and IMF conditions such thermalization might take place in limited regions of Mercury's magnetosphere, primarily in the sub-solar magnetosheath. Due the small scale size of Mercury's magnetosphere compared to the gyro-radii of these heavy ions and their associated wave modes, hybrid simulation with a kinetic treatment for the ions and a fluid treatment for the electrons may be the only way to study if thermalization of Na+ can occur. Preliminary results of a hybrid simulation that incorporates the Na+ pickup ions in its kinetic treatment will be presented.
SM53C-1411
Development of a MHD code satisfying solenoidal magnetic field condition and its application to Mercury's magnetosphere
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
SM53C-1412
Structure of Mercury's magnetosphere for different solar wind beta: three dimensional hybrid simulations
Recent results of 3D hybrid simulations of Mercury's magnetosphere revealed its basic structure with well pronounced cusp regions and also a closed ion ring forms around the planet. In general the plasma within the magnetosphere is more energetic in the high solar wind pressure case and the ion foreshock contains hotter magnetosheath plasma. Particles originating from the planet disperse throughout the magnetosphere, with the greatest congregation occuring in the inner magnetospheric drift-driven rings in both cases. These planetary particles can also leak upstream into the foreshock region. We study changes in properties and structure of the Mercury's magnetosphere with different solar wind beta, in particular the magnetosheath. We also predict values of basic plasma parameters obtained during the flyby of MESSENGER in January 2008.