[P-CG21] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG21-P05] Non-thermal escape of expanded hydrogen atmosphere on ancient Earth modeled by 3D multi-ion component magnetohydrodynamic simulation

*Yuri Kusano¹, Tomoki Kimura¹, Shotaro Sakai², Tatsuya Yoshida², Yuki Maeda³, Eitaro Nakada⁴, Ryoma Takada¹, Minami Tokushige¹, Naoki Terada² (1.Tokyo University of Science, 2.Tohoku University, 3.The University of Tokyo, 4.Hokkaido University)

Keywords:ancient Earth, non-thermal escape

The most important element for Earth and other planets to hold oceans on the surface is the atmosphere. The atmospheric evolution has been controlled by the supply of gas from Earth's crust and its escape into space. Non-thermal atmospheric escape, such as ion pickup, which is one of the major processes of atmospheric escape, depends on the planet's intrinsic magnetic field, the solar wind, and solar XUV flux. Quantitative evaluation and long-term variability of each process are important for understanding the atmospheric evolution, but there is a lack of in-situ observations of the escape regions of each planet, which is a major unsolved problem. Previous studies have estimated the non-thermal escape rates from the planetary atmospheres such as Mars and Venus, using the global numerical simulations. For example, the atmospheric escape simulation of past Mars have shown that when the XUV flux increases 100 times compared to the present day, the atmospheric temperature (Kulikov et al., 2007) and ion production rate increase, and the escape rate increases by 10^4-5 times (Terada et al., 2009). However, these studies are based on estimates of the composition of the present neutral atmosphere and are based on the assumption that the ancient Martian atmosphere was CO₂-rich and dry with a low greenhouse effect as well as the present Mars. On the other hand, the ancient atmosphere likely had a hydrogen-dominant atmosphere (Yoshida and Kuramoto., 2021), which is different from the present. A similar problem remains unresolved on Earth. Here we hypothesized a hydrogen atmosphere of the ancient Earth (Yoshida and Kuramoto., 2021, altitude range of 1000~185000km, the maximum density of 3×10¹²/cm³), assuming an XUV flux 100 times larger than present, and investigated the non-thermal escape rate of atmospheric ions using a multispecies ion magnetohydrodynamic (MHD) model (Terada et al., 2009). For simplicity, Earth was set to be non-magnetic, the solar wind with a speed of 1800 km/s, a density of 2100/cc, and an IMF absolute value of 7 nT. As a result, a bow shock was formed at about 60 R_p on the dayside (R_p is the radius of the Earth 6380 km), and a region that would be called the "expanded ionosphere" was formed at < 60 R_p with the maximum plasma density of 5.4×10⁶/cc due to the expanded atmosphere. The solar wind magnetic field penetrated into the expanded atmosphere and piled up on the dayside, consequently forming an induced magnetosphere with a maximum magnetic flux density of ~2.0×10³ nT. The magnetic flux density of the IMF is amplified by a factor of 400 in the expanded ionosphere. The atmospheric escape rate at 200 R_p on the night side is found to be 4.0×10²⁸/s for H₂⁺ and 5.0×10³³/s for H⁺. That is about 7 orders of magnitude (H⁺) greater than the present value. In the future, we plan to investigate the dependence of atmospheric escape rate on the solar XUV flux by changing XUV from 1 to 100 times greater than the present under the same simulation conditions.