Cross-scale energy transfer from fluid-scale Alfv\'en waves to kinetic-scale ion acoustic waves in the Earth's magnetopause boundary layer

TL;DR

This study provides observational and computational evidence that large-amplitude Alfvén waves in the Earth's magnetopause can transfer energy to ion acoustic waves at kinetic scales, leading to plasma heating and ion acceleration.

Contribution

It demonstrates a novel energy transfer mechanism from fluid-scale Alfvén waves to kinetic-scale ion acoustic waves in space plasmas, validated by combined observations and simulations.

Findings

Alfvén waves drive compressive ion acoustic waves at kinetic scales.

Ion populations are accelerated and heated near ion acoustic velocities.

Energy transfer from fluid to kinetic scales facilitates plasma heating.

Abstract

In space plasmas, large-amplitude Alfv\'en 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 channelling 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 Alfv\'en 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 Alfv\'en waves, not only accelerate the ion tail population to the Alfv\'en velocity, but also heat the ion core population near the ion acoustic velocity and generate Debye-scale ion acoustic waves. Thus, Alfv\'en-acoustic energy channeling emerges as a viable mechanism for plasma heating near plasma boundaries where large-amplitude Alfv\'en waves are present.