Quantification of non-Maxwellian properties in plasma mixing during magnetopause reconnection

We investigate ion velocity-space dynamics within the exhaust region of asymmetric magnetopause reconnection using global hybrid-Vlasov simulations. To quantify the complexity of velocity-space structures arising from the mixing of magnetospheric and magnetosheath ion populations, we employ the Hermite transform and Gaussian Mixture Model (GMM) analyses. In the Hermite representation, we use a fixed number of 22 harmonics to ensure computational feasibility. From this expansion, we compute a scalar measure of enstrophy—the total power contained in the non-zero Hermite modes—which characterizes the available free energy in the system. For the GMM approach, we test different numbers of ion populations and evaluate the corresponding multi-beam thermal energy for each decomposition. We further define the thermal energy drop as the relative difference between the thermal energy of an equivalent single-Maxwellian distribution and the total multi-beam thermal energy. Both enstrophy and thermal energy drop diagnostics (for any number of beams considered) exhibit consistent trends during the phase of plasma thermalization and anisotropic acceleration, demonstrating that the redistribution of thermal energy can be effectively captured even with a limited number of Hermite modes.