Multi-scale Dynamics of Coherent Electron Trapping and Diffusion in Earth's Magnetosheath

Space and astrophysical plasmas exhibit electromagnetic fluctuations and inhomogeneous structures across a wide range of scales. In the turbulent magnetosheath, high-frequency whistler waves are closely associated with large-scale coherent structures such as magnetic holes. Our study presents statistical evidence on the generation and diffusion efficiency of two distinct groups of whistler modes. Temperature-anisotropy and beam-type instabilities are triggered at different stages of magnetic hole evolution. We introduce a quasi-linear model demonstrating the crucial role of adiabatic trapping and cooling of electrons in generating these whistler waves. As the magnetic hole steepens, the slow evolution of unstable electron velocity distribution functions indicates a transition from temperature-anisotropy to beam-type instabilities, which reach saturation at faster time scales. This multi-scale mechanism offers new insights into the excitation and dissipation of whistler-mode fluctuations in similar environments.