[P-CG18] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG18-05] Ion escape mechanism from a Mars-like planet under weak intrinsic magnetic field conditions: Dependence of IMF clock angle

*Shotaro Sakai^1,2, Kanako Seki³, Naoki Terada¹, Hiroyuki Shinagawa⁴, Ryoya Sakata³, Takashi Tanaka^4,5, Yusuke Ebihara⁶ (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 3.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 4.National Institute of Information and Communications Technology, 5.International Center for Space Weather Science and Education, Kyushu University, 6.Research Institute for Sustainable Humanosphere, Kyoto University)

Keywords:Mars, Atmospheric escape, Intrinsic magnetic field, Interplanetary magnetic field, Numerical modeling

The ion escape mechanism from planets is mainly determined by the presence of an intrinsic magnetic field, the orientation of the interplanetary magnetic field (IMF), and the solar X ray and extreme ultraviolet (XUV) irradiances. It is believed that ancient Mars had a global intrinsic magnetic field of interior origin and the magnetic field decayed by ~3.9 billion years ago (Acuña et al., 1999). One of the pieces of evidence that ancient Mars had an intrinsic field is the existence of a "crustal magnetic field” (Acuña et al., 1999). Sakai et al. (2018) investigated the effect of a weak intrinsic magnetic field at the Martian equatorial surface on the escape mechanism. It was shown that the existence of the weak field results in an enhancement of the ion escape rate. A parker-spiral IMF was however used in order to obtain the escape rate in this earlier study. The orientation of IMF also changes the escape rate and mechanism. Sakai et al. (2021) suggested that the escape rate is the lowest in the case of IMF parallel to the dipole at subsolar and comparable in the Parker-spiral and antiparallel IMF case. The parallel IMF case is dominated by the ion escape from the high-latitude lobe reconnection region, where ionospheric ions are transported upward along open field lines, while in the antiparallel IMF case, the escape flux of heavy ions increases significantly due to the mass loading of ionospheric ions, with peaks around the equatorial dawn and dusk flanks. Previous studies have investigated the ion escape rate and its mechanism for only three IMF orientations, e.g., the northward (parallel), Parker-spiral, and southward (antiparallel) IMF cases, but the factors responsible for the enhancement of escape rate have not yet been elucidated.

This paper investigates the dependence of the escape mechanism on the IMF clock angle under a weak intrinsic magnetic field of 100 nT at the surface and the present-day XUV irradiances using a multispecies magnetohydrodynamic simulation. The simulation is conducted under the condition that the IMF makes a round in twenty-four hours, which is comparable to the time scale of coronal mass ejection, after starting the northward IMF. The ion escape rate drastically increases when the clock angle is above 45^o from the due north. The reconnections occur in the lobe region of the magnetosphere in the northward IMF, its location gradually moving to the flank region during the transition of the clock angle to 45^o. The reconnections in the flank region also occur in the Parker-spiral IMF case (Sakai et al., 2018; 2021), leading to the enhancement of the ion escape rate. The escape rate gradually increases over time after the clock angle of 60^o, reaching a maximum around the clock angle of 180^o - 210^o in the southward IMF.

This paper also studies the escape mechanism under the different magnetic field strengths and XUV irradiances influencing the escape mechanism, and the results are compared with those under the magnetic field strength of 100 nT and the present-day XUV irradiances.

References:
Acuña, M., Connerney, J. E. P., Ness, N. F., Lin, R. P., Mitchell, D., Carlson, C. W., et al. (1999). Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER experiment, Science, 284, 790-793. https://doi.org/10.1126/science.284.5415.790
Sakai, S., Seki, K., Terada, N., Shinagawa, H., Tanaka, T., & Ebihara, Y. (2018). Effects of a weak intrinsic magnetic field on atmospheric escape from Mars. Geophys. Res. Lett., 45, 9336-9343. https://doi.org/10.1029/2018GL079972
Sakai, S., Seki, K., Terada, N., Shinagawa, H., Sakata, R., Tanaka, T., & Ebihara, Y. (2021). Effects of the IMF direction on atmospheric escape from a Mars-like planet under weak intrinsic magnetic field conditions. J. Geophys. Res. Space Physics, in revision.