Energy conversion pathways inside Kelvin-Helmholtz vortices

At the Earth's magnetopause, velocity shear between the fast-flowing magnetosheath plasma and the stagnant magnetosphere can drive the Kelvin—Helmholtz instability (KHI), a multi-scale process generating vortices that transport mass, momentum, and energy across the boundary, thus contributing to plasma mixing and solar wind—magnetosphere coupling. Although in-situ observations and simulations have linked KHI to turbulence onset and plasma heating, the precise pathways of energy transfer remain unexplored. By leveraging high-resolution, multi-point Magnetospheric Multiscale (MMS) observations, we quantify, for the first time, KHI energy conversion channels via pressure—strain and J· E' diagnostics. Inside vortices, thermal-to-flow energy transfer dominates, coinciding with local non-thermal features. Notably, energy pathways depend on vortex evolution: rolled-up vortices convert flow energy into thermal energy, whereas early-stage vortices convert thermal energy back into flow. This study provides the first energy-balance-based description of KHI dynamics and further reveals the complexity of velocity shear energy dissipation into heat.