Cross‐Scale Coupling in Earth's Dayside Magnetosphere 2D Semi‐Implicit PIC Simulation: From Global Geometry to Multi‐Scale Kinetic Dynamics

Abstract The interaction between the solar wind and the terrestrial magnetosphere is a complex, multiscale process where global‐scale dynamics are fundamentally regulated by microscopic kinetic physics. We present a global two‐dimensional semi‐implicit particle‐in‐cell (PIC) simulation of the Earth's dayside magnetosphere using the FLEKS model, bridging the gap between fluid‐scale magnetohydrodynamics and kinetic‐scale plasma processes. The interaction of the solar wind with the terrestrial dipole is modeled with fully kinetic ions and electrons (mass ratio ), capturing the self‐consistent formation of the foreshock, bow shock, and magnetosheath. The system evolves from an initial MHD state to a complex kinetic equilibrium characterized by a reforming quasi‐parallel shock and a granular magnetosheath. We identify a causal link between foreshock‐generated ultra‐low frequency (ULF) waves, which propagate downstream, and the spatial organization of the magnetosheath, where anisotropy‐driven Alfvén ion cyclotron and mirror‐mode waves coexist with broadband magnetic turbulence. Particle tracking reveals that while protons are energized via a combination of Fermi acceleration and shock drift acceleration in the foreshock, backstreaming electrons are predominantly leakage particles from the thermalized magnetosheath. These results demonstrate that semi‐implicit PIC simulations can effectively resolve cross‐scale coupling mechanisms, such as modification of global shock geometry by kinetic instabilities, providing a comprehensive view of the collisionless dayside solar wind‐magnetosphere interaction.