Mirror modes and magnetic holes in Mercury’s magnetosheath. Nonlinear evolution under turbulent solar-wind conditions

Mirror-mode (MiMo) fluctuations and magnetic holes (MHs) are commonly observed in collisionless space plasmas. At Mercury, MHs are frequently detected in the magnetosheath, whereas clear observations of MiMos remain comparatively rare, raising questions about the relation between these two types of structures and the conditions under which MHs form. We investigated the formation mechanisms and physical properties of MHs in Mercury's magnetosheath and assessed the role of upstream solar-wind turbulence and bow-shock configuration in their formation. We analyzed two global 3D hybrid-Particle-in-cell (PIC) simulations of Mercury’s magnetosphere performed with the Menura solver, considering both laminar and turbulent upstream solar-wind conditions. For laminar upstream conditions, the quasi-perpendicular magnetosheath exhibits MiMo activity, but no isolated MHs form. In contrast, under turbulent solar-wind conditions, MiMos generated downstream of the bow shock evolve nonlinearly into proton-scale MHs. A case study shows that during their early evolution, these structures are characterized by local pressure balance, enhanced ion trapping, and mirror-unstable plasma conditions. Furthermore, a statistical analysis of 72 MHs shows that the structures are typically elongated along the magnetic field and remain close to pressure balance. We detect only negligible MH activity upstream in the solar wind, indicating that the structures form locally within the magnetosheath after solar-wind turbulence affects the shocked region. Solar-wind turbulence plays a key role in MH formation at Mercury by disrupting coherent MiMo activity downstream of the quasi-perpendicular bow shock. These results provide new constraints for interpreting MH observations at Mercury and predictions relevant to the BepiColombo mission.