10:15 〜 10:30
[PCG18-06] Dependences of ion escape from Mars on the intrinsic magnetic field strength under CME-like solar conditions at 3.5 Ga
キーワード：大気散逸、固有磁場、MHDシミュレーション、太古火星
Mars experienced drastic climate change during its early period. Atmospheric escape to space played an important role in the removal of the atmosphere and climate change on ancient Mars. In particular, escape of the ionized atmosphere, i.e., ion escape was a key process of atmospheric loss. Stronger solar X-ray and extreme ultraviolet (XUV) radiation and solar wind from the young Sun facilitate ion escape. Past studies pointed out that ion escape on ancient Mars was stronger by several orders of magnitude and contributed to the atmospheric loss. A planetary magnetic field is another important parameter controlling ion escape. Mars has crustal remnant magnetic field, which implies that ancient Mars once had a global intrinsic magnetic field. Existence of an intrinsic magnetic field affects the magnetic configuration and thus processes and rates of ion escape. Although previous studies suggested both inducing and reducing effects of an intrinsic magnetic field on ion escape, the details are not fully understood. Based on multispecies magnetohydrodynamics (MHD) simulations, Sakata et al. (2020) investigated how the strength of an intrinsic magnetic field affects ion escape from Mars under the solar XUV and solar wind conditions at 4.5 Ga. They revealed that the effects of an intrinsic magnetic field depend on the relationship between the solar wind dynamic pressure and the magnetic pressure of the intrinsic magnetic field and showed that the effects are more pronounced on molecular ions (O2+ and CO2+). Escape rates of molecular ions increase by a factor of six if Mars has a weak intrinsic magnetic field whose magnetic pressure is lower than the solar wind dynamic pressure, while they decrease by two orders of magnitude if the intrinsic magnetic field is strong enough to sustain the solar wind dynamic pressure. Effects on O+ are milder than those on molecular ions because of the contribution of ion pickup on the extended neutral oxygen corona.
We investigated dependences of ion escape processes and rates on the intrinsic magnetic field strength under solar XUV and solar wind conditions during coronal mass ejection (CME)-like events at 3.5 Ga. As solar events such as CMEs occurred more frequently during the early period of the Sun, enhanced ion escape during CMEs is expected to play a role in atmospheric loss from ancient Mars. The solar XUV flux was set to be 50 times higher than the present value. The solar wind number density, velocity, and the interplanetary magnetic field (IMF) strength were set to be 700 cm-3, 1400 km s-1, and 20 nT, respectively. The intrinsic magnetic field was assumed to be a dipole field and northward at the equator. We conducted multiple simulations cases with different strength of the dipole field.
As expected, escape rates of molecular ions increase in the weak dipole field case, while they decreased strongly under the existence of a strong dipole field enough to sustain the solar wind dynamic pressure. In the intermediate case, the escape rates decrease despite the dipole field strength is lower than the threshold defined so that the dipolar magnetic pressure at the equatorial surface equals the solar wind dynamic pressure. It suggests that the relationship between the intrinsic magnetic field and the solar wind should be evaluated more carefully based on the ion escape mechanism. The effects on O+ are seen more clearly than those in Sakata et al. (2020). The contribution of ion pickup on the neutral oxygen corona is smaller due to weaker solar XUV flux.
References
Sakata, R., et al. (2020). Effects of an intrinsic magnetic field on ion loss from ancient Mars based on multispecies MHD simulations. J. Geophys. Res., 125, e2019JA026945. doi:10.1029/2019JA026945
Terada, N., et al. (2009). A three-dimensional, multispecies, comprehensive MHD model of the solar wind interaction with the planet Venus. J. Geophys. Res, 114(9), 1–11. doi:10.1029/2008JA013937
We investigated dependences of ion escape processes and rates on the intrinsic magnetic field strength under solar XUV and solar wind conditions during coronal mass ejection (CME)-like events at 3.5 Ga. As solar events such as CMEs occurred more frequently during the early period of the Sun, enhanced ion escape during CMEs is expected to play a role in atmospheric loss from ancient Mars. The solar XUV flux was set to be 50 times higher than the present value. The solar wind number density, velocity, and the interplanetary magnetic field (IMF) strength were set to be 700 cm-3, 1400 km s-1, and 20 nT, respectively. The intrinsic magnetic field was assumed to be a dipole field and northward at the equator. We conducted multiple simulations cases with different strength of the dipole field.
As expected, escape rates of molecular ions increase in the weak dipole field case, while they decreased strongly under the existence of a strong dipole field enough to sustain the solar wind dynamic pressure. In the intermediate case, the escape rates decrease despite the dipole field strength is lower than the threshold defined so that the dipolar magnetic pressure at the equatorial surface equals the solar wind dynamic pressure. It suggests that the relationship between the intrinsic magnetic field and the solar wind should be evaluated more carefully based on the ion escape mechanism. The effects on O+ are seen more clearly than those in Sakata et al. (2020). The contribution of ion pickup on the neutral oxygen corona is smaller due to weaker solar XUV flux.
References
Sakata, R., et al. (2020). Effects of an intrinsic magnetic field on ion loss from ancient Mars based on multispecies MHD simulations. J. Geophys. Res., 125, e2019JA026945. doi:10.1029/2019JA026945
Terada, N., et al. (2009). A three-dimensional, multispecies, comprehensive MHD model of the solar wind interaction with the planet Venus. J. Geophys. Res, 114(9), 1–11. doi:10.1029/2008JA013937