Dynamics in Mercury’s magnetospheric cusps: New insights from global coupled fluid-kinetic simulations

MESSEGER observations at Mercury have uncovered a highly dynamic magnetosphere, primarily driven by magnetic reconnection between the IMF and the planet’s intrinsic field. Frequent reconnection at Mercury’s dayside magnetopause efficiently couples the solar wind to the magnetosphere, resulting in the acceleration and heating of solar wind particles as they enter the magnetosphere. In this study, we present findings from global coupled fluid-kinetic simulations of Mercury’s magnetosphere (Li et al., 2024, JGR), focusing on the dynamics in the cusp region. These simulations utilize the MHD-AEPIC (MHD with Adaptively Embedded PIC) model, conducted with varying solar wind and IMF conditions. Our results reveal frequent occurrences of filamentary structures in the cusp region, marked by significant reductions in magnetic field strength and notable increases in plasma density and pressure. These features closely resemble those of the so-called “cusp filaments” observed by MESSENGER (e.g., Poh et al., 2016, JGR). By tracking their evolution in our simulations, we demonstrate that cusp filaments essentially represent the high-latitude extensions of flux transfer events (FTEs). Additionally, we examine the spatial distribution and variability of solar wind proton precipitation onto the planetary surface through the cusps, which is crucial for understanding surface weathering and exosphere generation at Mercury. Our simulations indicate global precipitation rates ranging from 0.7 to 2.5×10²⁵ particles per second, consistent with MESSENGER observations, and these rates increase with decreasing solar wind Mach number and IMF clock angle. Moreover, we observe a notable dawn-dusk asymmetry in proton precipitation patterns, with enhanced dawnside precipitation, aligning with asymmetries predicted in magnetopause reconnection occurrences by previous MHD-AEPIC simulations.