Effects of the IMF direction on atmospheric escape from Mars under weak intrinsic magnetic field conditions

Abstract:

The planetary intrinsic magnetic field is critical when considering the atmospheric escape from planets. The strength of the intrinsic magnetic field particularly affects the interaction between solar wind and terrestrial-type planets and it changes the escape mechanism. The direction of the upstream interplanetary magnetic field (IMF) also significantly changes the magnetospheric configuration, influencing the atmospheric escape rate and mechanism. This paper investigates the effects of the IMF direction on the ion escape mechanism from a Mars-like planet that has a northward weak intrinsic magnetic field on the equator. The northward, southward, and Parker-spiral IMFs under present solar wind conditions are compared based on multispecies magnetohydrodynamics simulations. In the northward IMF case, molecular ions escape from the high-latitude lobe reconnection region, where ionospheric ions are transported upward along open field lines. Oxygen ions originating either in the ionosphere or oxygen corona escape through a broader ring-shaped region. In the southward IMF case, the escape flux of heavy ions increases significantly and has peaks around the equatorial dawn and dusk flanks. The draped IMF can penetrate into the subsolar ionosphere by erosion, and the IMF becomes mass-loaded as it is transported through the dayside ionosphere. The mass-loaded draped IMF is carried to the tail, contributing to ion escape. The escape channels in the northward and southward IMF cases are different from those in the Parker-spiral IMF case. The escape rate is the lowest in the northward IMF case and comparable in the Parker-spiral and southward IMF cases. In the northward IMF case, a weak intrinsic dipole forms a magnetosphere configuration similar to that of Earth, quenching the escape rate, while the Parker-spiral and southward IMFs cause reconnection and erosion, promoting ion escape from the upper atmosphere.