Cross-Scale Energy Transfer from Fluid-Scale Alfvén Waves to Kinetic-Scale Ion Acoustic Waves in the Earth’s Magnetopause Boundary Layer

In space plasmas, large-amplitude Alfvén waves can drive compressive perturbations, accelerate ion beams, and lead to plasma heating and the excitation of ion acoustic waves at kinetic scales. This energy channeling from fluid to kinetic scales represents a complementary path to the classical turbulent cascade. Here, we present observational and computational evidence to validate this hypothesis by simultaneously resolving the fluid-scale Alfvén waves, kinetic-scale ion acoustic waves, and their imprints on ion velocity distributions in the Earth's magnetopause boundary layer. We show that two coexisting compressive modes, driven by the magnetic pressure gradients of Alfvén waves, not only accelerate the ion tail population to the Alfvén velocity, but also heat the ion core population near the ion acoustic velocity and generate Debye-scale ion acoustic waves. Thus, Alfvén-acoustic energy channeling emerges as a viable mechanism for plasma heating near plasma boundaries where large-amplitude Alfvén waves are present.