Solar wind- Mercury's magnetosphere interaction by data exploration and MHD simulations

Mercury's magnetosphere is more dynamic than Earth's due to its proximity to the Sun, and it is subject to a lower Mach number solar wind. Regarding the solar wind interaction with Mercury, we are interested in the configurations of Mercury’s magnetosphere and the energy transport under various solar wind conditions. First, this study examines the potential impact of low Mach number solar wind on Mercury's bow shock and the resulting effects on the magnetosphere. To analyze the variability of Mercury's bow shock in response to solar wind properties, this study combines observations by the Helios data with theoretical solutions and MHD simulations. The results show that when Mercury encounters solar wind with an extremely low Mach number, its bow shock is expected to become more flattened, further from the planet, and may even disappear completely. Our other focus is on the Kelvin-Helmholtz instability (KHI) that occurs at the magnetopause, which plays a crucial role in the energy transfer and momentum coupling process between the solar wind and Mercury's magnetospheres. We conducted MHD simulations based on boundary conditions and plasma parameters from a global hybrid simulation of the MESSENGER’s first flyby in 2008. Given the lack of comprehensive plasma observations of Mercury's magnetosphere, we examined two scenarios: one with a heavily mass-loaded magnetosphere and another with a weakly mass-loaded magnetosphere. Our findings show that the KHI in a heavily loaded magnetosphere results in a more turbulent magnetopause, with nonlinear fast-mode plane waves expanding away from the magnetopause. The momentum and energy flux quantified from our simulations reveals that the KHI with a heavily loaded magnetosphere can efficiently transport momentum and energy away from the magnetopause in the presence of the fast-mode plane waves. In such a scenario, observed in the inner magnetosphere, the momentum flux can reach about 0.5 % of the initial solar-wind dynamic pressure; the energy flux can be 10^-2 erg/cm²/s, and the energy density is about 1.5 %-3.0 % of the initial solar-wind energy.