10:00 〜 10:15
[PCG19-05] Study on effects of an intrinsic magnetic field on ancient Mars with a new multifluid MHD model
キーワード：大気散逸、太古火星、多流体MHDシミュレーション
Escape of the ionized atmosphere, ion escape, played a role in atmospheric loss and climate change in ancient Mars because intense solar X-ray and EUV (XUV) radiation and solar wind from the young Sun enhance ion escape from the planet. On the other hand, ancient Mars had a global intrinsic magnetic field. It affects the magnetospheric configuration and thus ion escape, but the detailed effects on ion escape from ancient Mars are yet to be fully understood. Based on multispecies magnetohydrodynamics (MHD) simulations, Sakata et al. (2022) pointed out that the effects depend on the pressure balance between the solar wind dynamic pressure and the magnetic pressure of the dipole field. However, the representation of outflow from the ionosphere is insufficient in the previous study due to the model limitation. The multispecies MHD model is based on the single-fluid assumption and cannot represent inflow of solar wind ions and outflow of ionospheric ions simultaneously. Therefore, the outflow of ionospheric ions may be underestimated, particularly under the existence of a strong intrinsic magnetic field.
We developed a new three-dimensional global multifluid MHD model. It 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 that is characterized by a quasi-uniform horizontal grid and six faces with identical coordinate system. The simulation domain is from 100 km altitude to 40 planetary radii with the non-uniform vertical grid. The model includes important processes in the ionosphere: chemical reactions, photoionization, charge exchange, and collisions among ions, neutrals, and electrons. We conducted six multifluid MHD simulations with the dipole field strength of 0, 100, 500, 1000, 2000, and 5000 nT on the equatorial surface under ancient solar XUV and solar wind conditions used in Sakata et al. (2020). For comparison, the multispecies MHD simulations were also conducted under the same solar and dipole field conditions.
The global configuration of the magnetic field is similar between the multifluid and multispecies cases. However, the multifluid cases show asymmetric planetary ion distribution with a plume-like structure in the +E hemisphere due to the convection electric field of the solar wind. The escape rates of molecular ions are higher than those in the corresponding multispecies cases, indicating enhancement of outflow from the ionosphere. The enhancement is more than two orders of magnitude in the strongest dipole field case. The separation of the momentum equations allows planetary ions to flow out from the ionosphere independently of precipitating solar wind protons. The different plasma dynamics also change the composition of the ionosphere. The main driver of outflow is the electromagnetic force imposed by the solar-wind interactions. On the other hand, the escape rate of O+ is slightly lower in no or weak dipole field cases due to suppression of ion pickup in the –E hemisphere. In the strong dipole field cases, however, the O+ escape rate is one order of magnitude higher than in the corresponding multispecies cases because outflow from the ionosphere becomes dominant instead of ion pickup. The total escape rate of the multifluid case with no dipole field reaches the order of 1027 s-1 but is decreased by a factor of five in the strongest dipole field case.
References
Sakata, R., Seki, K., Sakai, S., Terada, N., Shinagawa, H., & Tanaka, T. (2022). Multispecies MHD study of ion escape at ancient Mars: Effects of an intrinsic magnetic field and solar XUV radiation. Journal of Geophysical Research: Space Physics, 127, e2022JA030427. https://doi.org/10.1029/2022JA030427
We developed a new three-dimensional global multifluid MHD model. It 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 that is characterized by a quasi-uniform horizontal grid and six faces with identical coordinate system. The simulation domain is from 100 km altitude to 40 planetary radii with the non-uniform vertical grid. The model includes important processes in the ionosphere: chemical reactions, photoionization, charge exchange, and collisions among ions, neutrals, and electrons. We conducted six multifluid MHD simulations with the dipole field strength of 0, 100, 500, 1000, 2000, and 5000 nT on the equatorial surface under ancient solar XUV and solar wind conditions used in Sakata et al. (2020). For comparison, the multispecies MHD simulations were also conducted under the same solar and dipole field conditions.
The global configuration of the magnetic field is similar between the multifluid and multispecies cases. However, the multifluid cases show asymmetric planetary ion distribution with a plume-like structure in the +E hemisphere due to the convection electric field of the solar wind. The escape rates of molecular ions are higher than those in the corresponding multispecies cases, indicating enhancement of outflow from the ionosphere. The enhancement is more than two orders of magnitude in the strongest dipole field case. The separation of the momentum equations allows planetary ions to flow out from the ionosphere independently of precipitating solar wind protons. The different plasma dynamics also change the composition of the ionosphere. The main driver of outflow is the electromagnetic force imposed by the solar-wind interactions. On the other hand, the escape rate of O+ is slightly lower in no or weak dipole field cases due to suppression of ion pickup in the –E hemisphere. In the strong dipole field cases, however, the O+ escape rate is one order of magnitude higher than in the corresponding multispecies cases because outflow from the ionosphere becomes dominant instead of ion pickup. The total escape rate of the multifluid case with no dipole field reaches the order of 1027 s-1 but is decreased by a factor of five in the strongest dipole field case.
References
Sakata, R., Seki, K., Sakai, S., Terada, N., Shinagawa, H., & Tanaka, T. (2022). Multispecies MHD study of ion escape at ancient Mars: Effects of an intrinsic magnetic field and solar XUV radiation. Journal of Geophysical Research: Space Physics, 127, e2022JA030427. https://doi.org/10.1029/2022JA030427