[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-P01] A 3D magnetohydrodynamic modeling for atmospheric escape from Earth during the geomagnetic reversal event

*Yuki Maeda^1,2, Tomoki Kimura¹, Minami Tokushige¹, Eitaro Nakada³, Shotaro Sakai⁴, Naoki Terada⁵ (1.Tokyo University of Science, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Graduate School of Environmental Science, Hokkaido University, 4.Department of Geophysics, Graduate School of Science, Tohoku University, 5.Graduate School of Science, Tohoku University)

Keywords:atmospheric escape, magnetosphere, geomagnetic reversal

Geomagnetic reversal events of Earth, which have occurred repeatedly in the past, likely altered the solar wind-magnetosphere-atmosphere interaction. The alteration of the interaction process has a significant impact on the terrestrial atmospheric escape particularly for the non-thermal escape, which strongly depends on the geomagnetic field strength. For long years, theoretical modeling studies of the solar wind-magnetosphere-atmosphere interactions have been conducted for the geomagnetic reversal events. For example, Wei et al. (2014) suggested that during geomagnetic reversals, direct interaction between the solar wind and the atmosphere increases the escape rate of oxygen ions from the current value of 10^25/s to 10^28/s-10^29/s. On the other hand, some studies argued that even during the geomagnetic reversals, the magnetic shielding by the magnetosphere against the solar wind is still effective and the escape rate does not increase dramatically (Gunell et al., 2018; Tsareva et al., 2020). Most of the previous studies used analytical models or low-dimensional numerical models to describe the non-thermal escape. There has been no realistic numerical simulation that reproduces 3D structures. In this study, A three-dimensional, multispecies, one-fluid magnetohydrodynamics (MHD) simulation (Terada et al., 2000) was applied for the first time to the non-thermal atmospheric escape of Earth during the geomagnetic reversal event. We modeled the escape for different planetary dipole field strengths with an atmospheric profile of a mixture of nitrogen and oxygen, the solar X-ray and extreme ultraviolet (XUV) radiation flux 100 times higher than the present-day, and relatively calm solar wind conditions (a solar wind flow speed of 450 km/s, plasma density of 7/cc, temperature of 1.2 x 10^5 K, and IMF field strength of 10 nT). As the planetary dipole magnetic field strength at the equatorial surface increases from 0 nT to 2000 nT, the escape rate monotonically increases from 5.1 x 10^-1 kg/s (1.9 x 10^25/s) to 2.1 x 10^1 kg/s (7.9 x 10^26/s). We concluded that Earth's intrinsic magnetic field may ‘activate’ the non-thermal escape rather than suppress it. It is also found that for the stronger dipole field cases, a flow channel was formed in the polar cap region, along which the atmospheric ion components escape anti-planetward. A force balance analysis showed that at low altitudes in the flow channel, the escape is mainly driven by the plasma pressure gradient, while at higher altitudes, it is driven by the magnetic pressure gradient force and magnetic tension force.