Global Hall Magnetohydrodynamics and Coupled Fluid-Kinetic Simulations of Mercury's Dayside Magnetopause Dynamics

Abstract

Mercury possesses a miniature yet dynamic magnetosphere driven primarily by the solar wind through magnetic reconnection. Because Mercury is 60% closer to the Sun than Earth, its magnetosphere routinely experiences stronger external drivings than typically seen at Earth, thereby providing a natural laboratory for comparative studies of magnetic reconnection and the resultant magnetospheric dynamics. A prominent feature of Mercury’s interaction with the solar wind at the dayside magnetopause is frequent occurrence of flux transfer events (FTEs), which are thought to play an important role in driving the global convection. To investigate the generation and characteristics of FTEs under different solar wind Alfvénic Mach numbers (MA) and IMF orientations, we first conducted a series of global simulations using the BATSRUS Hall Magnetohydrodynamics (MHD) model, which facilitates the occurrence of fast reconnection in current sheets by allowing separate bulk motions of plasma ions and electrons. An automated algorithm was also developed to consistently identify FTEs and extract their key properties from the simulations. In all simulations driven by steady upstream conditions, FTEs are formed quasi-periodically with recurrence time ranging from 2 to 9 seconds, and their characteristics vary in time as they evolve and interact with the surrounding environment. Our statistical analysis of the simulated FTEs reveals that the key properties of FTEs, including spatial size, traveling speed and core field strength, all exhibit notable dependence on the solar wind MA and IMF orientation, and the trends identified from the simulations are generally consistent with previous MESSENGER observations. It is also found that FTEs formed in the simulations contribute about 3%-13% of the total open flux created at the dayside magnetopause that participates in the global circulation. Next, we performed a series of Magnetohydrodynamics with Adaptively Embedded Particle-in-Cell (MHD-AEPIC) simulations using the same input parameters as used in the Hall-MHD runs to study in detail the kinetic signatures, asymmetries, and FTEs associated with Mercury's dayside magnetopause reconnection. By treating both ions and electrons kinetically, the embedded PIC model reveals crescent-shaped phase-space distributions near reconnection sites, counter-streaming ion populations in the cusp region, and temperature anisotropies within FTEs. A novel metric and algorithm are developed to automatically identify reconnection X-lines in our 3D simulations. The spatial distribution of reconnection sites as modeled by the PIC code exhibits notable dawn-dusk asymmetries, likely due to kinetic effects such as X-line spreading and Hall effects. The properties of FTEs in the MHD-AEPIC simulations also show clear dependencies on the solar wind MA and IMF orientation, consistent with MESSENGER observations and previous Hall-MHD simulations. FTEs formed in our MHD-AEPIC model tend to carry a large amount of open flux, contributing 3%-36% of total open flux generated at the dayside, suggesting that FTEs indeed play an important role in driving the Dungey cycle at Mercury. Finally, we further analyzed the MHD-AEPIC simulation results to investigate the properties of simulated cusp filaments and proton precipitation, both of which have important consequences on the space weathering at Mercury. Our analysis reveals that cusp filaments map directly to FTEs and the ions and electrons within cusp filaments are significantly energized by the magnetopause reconnection. Dawn-dusk asymmetries in proton precipitation are also observed in close correspondence to the asymmetries in magnetopause reconnection occurrence. The global proton precipitation rate is found to increase with decreasing solar wind MA and decreasing IMF clock angle.