Cross-scale energy transport in space plasmas

The solar wind is a supersonic magnetized plasma streaming far into the heliosphere. Although cooling as it flows, it is rapidly heated upon encountering planetary obstacles. At Earth, this interaction forms the magnetosphere and its sub-regions. The present paper focuses on particle heating across the boundary separating the shocked solar wind and magnetospheric plasma, which is driven by mechanisms operating on fluid, ion and electron scales. The cross-scale energy transport between these scales is a compelling and fundamental problem of plasma physics. Here, we present evidence of the energy transport between fluid and ion scales: free energy is provided in terms of a velocity shear generating fluid-scale Kelvin–Helmholtz instability. We show the unambiguous observation of an ion-scale magnetosonic wave packet, inside a Kelvin–Helmholtz vortex, with sufficient energy to account for observed ion heating. The present finding has universal consequences in understanding cross-scale energy transport, applicable to environments experiencing velocity shears during comparable plasma regimes. Processes in (space) plasmas occur on different levels — fluid, ion and electron. Now, from satellite data and simulations, an energy-transfer mechanism between the fluid and ion scales is reported: fluid velocity shear is converted into ion heating.