[P-EM16] 宇宙プラズマ理論・シミュレーション

[PEM16-12] Simulations of ion escape at ancient Mars based on a new multifluid MHD model with the cubed sphere

*坂田 遼弥¹、関 華奈子¹、堺 正太朗^2,3、品川 裕之⁴、寺田 直樹² (1.東京大学大学院理学系研究科地球惑星科学専攻、2.東北大学理学研究科地球物理学専攻、3.東北大学理学研究科惑星プラズマ・大気研究センター、4.国立研究開発法人情報通信研究機構)

キーワード：大気散逸、太古火星、MHDシミュレーション

Mars had experienced intense atmospheric escape to space due to strong solar X-ray and EUV (XUV) radiation and solar wind during the early period of the Sun. However, the presence of an intrinsic magnetic field on ancient Mars affected ion escape, i.e., escape of the ionized atmosphere, because it changes the electromagnetic configuration around the planet. The effect of an intrinsic magnetic field on ion escape is still on debate. Based on global multispecies magnetohydrodynamics (MHD) simulations, Sakata et al. (2020) pointed out that the effects of an intrinsic magnetic field depend on the pressure balance between the solar wind dynamic pressure and the magnetic pressure of the intrinsic magnetic field, and that the effects are more remarkable on escape of molecular ions such as O₂⁺ and CO₂⁺ than on O⁺ escape. The main process of molecular ion escape is outflows from the ionosphere, while O⁺ escape is also due to ion pickup on the extended oxygen corona. These results suggest that the effects of an intrinsic magnetic field are different among ion species and/or escape processes. However, multispecies MHD simulations assume the same velocity on all ion species and cannot depict behavior of each ion species sufficiently. Since the outflow of ionospheric ions often occurs in a region where the solar wind H⁺ inflow and the planetary ion outflow coexist (e.g., the cusp region), different dynamics among ion species should play an important role.
We developed a new 3D global multifluid MHD model. The model solves the continuity, momentum, and energy equations of five ion species (solar wind H⁺, planetary H⁺, O⁺, O₂⁺, and CO₂⁺), the induction equation of the magnetic field, and the electron pressure equation. The model is implemented with the cubed sphere grid system, which has nearly uniform grid and no singular points (e.g., poles). The simulation domain is set to be from 100 km altitude to seven planetary radii with the non-uniform vertical grid. It includes chemical reactions, photoionization, charge exchange, and collisions (ion-ion, ion-electron, ion-neutral, and electron-neutral) necessary for the ionosphere. We adopted the composite diffusive/Powell method (Liu et al., 2021) for the divergence cleaning.
We conducted a 3D multifluid MHD simulation assuming the solar XUV and solar wind conditions at 3.5 Ga during a CME-like event. The solar wind density and velocity were 700 cm^-3 and 1400 km s^-1, respectively. The interplanetary magnetic field was away-sector Parker spiral, and its strength was 20 nT. The solar XUV radiation was 50 times higher than the current value. The planet had no intrinsic magnetic field. For comparison, we also conducted a multispecies MHD simulation under the same conditions. The results show that the tailward fluxes of molecular ions are asymmetric about the motional electric field of the solar wind in the multifluid simulation, while the fluxes are symmetric in the multispecies simulation. We focus on the molecular ion escape due to the ionospheric outflow and show the detailed analysis.

References
Liu, C., Shen, F., Liu, Y., Zhang, M., & Liu, X., (2021), Numerical Study of Divergence Cleaning and Coronal Heating/Acceleration Methods in the 3D COIN-TVD MHD Model. Front. Phys. 9:705744. doi:10.3389/fphy.2021.705744
Sakata, R., Seki, K., Sakai, S., Terada, N., Shinagawa, H., & Tanaka, T. (2020), Effects of an intrinsic magnetic field on ion loss from ancient Mars based on multispecies MHD simulations. Journal of Geophysical Research: Space Physics, 125, e2019JA026945. doi:10.1029/2019JA026945