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Global structure and sodium ion dynamics in Mercury's magnetosphere with the offset dipole

Abstract : We conducted global magnetohydrodynamics (MHD) simulation of Mercury's magnetosphere with the dipole offset, which was revealed by MESSENGER observations, in order to investigate its global structure under northward interplanetary magnetic field (IMF) conditions. Sodium ion dynamics originating from the Mercury's exosphere is also investigated based on statistical trajectory tracing in the electric and magnetic fields obtained from the MHD simulations. The results reveal a north-south asymmetry characterized by open field lines around southern polar region, and northward deflection of the plasma sheet in the far tail. The asymmetry of magnetic field structure near the planet drastically affects trajectories of sodium ion, and thus, their pressure distributions and precipitation pattern onto the planet. Weaker magnetic field strength in the southern hemisphere than in the north increases ion loss by precipitation onto the planetary surface in the southern hemisphere. The ‘sodium ring', which is formed by high-energy sodium ions drifting around the planet, is also found in the vicinity of the planet. The 'sodium ring' is almost circular under nominal solar wind conditions. The ring becomes partial under high solar wind density, because dayside magnetosphere is so compressed that there is no space for the sodium ions to drift around. In both cases, the 'sodium ring' is formed by sodium ions that are picked up and accelerated in the magnetosheath just outside the magnetopause and reentered into the magnetosphere due to combined effects of finite Larmor radius and convection electric field in the dawn-side magnetosphere.
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Manabu Yagi, Kanako Seki, Y. Matsumoto, Dominique C. Delcourt, François Leblanc. Global structure and sodium ion dynamics in Mercury's magnetosphere with the offset dipole. Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (11), pp.10,990-11,002. ⟨10.1002/2017JA024082⟩. ⟨insu-01585106⟩



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