Direct observations of cross-scale energy transfer driven by multiple-ion interactions in space plasmas

Astrophysical and space plasmas are inherently multi-scale in nature. Identifying the mechanisms responsible for transporting energy across different physical scales is an essential step in modeling the dynamics of these plasmas and their associated astrophysical and space systems. Traditionally, these mechanisms are examined within an electron-single ion framework, even in the presence of multiple ion species. This simplification is justified by the dominance of a single ion species in most observations. In this work, we present measurements from the Magnetospheric Multiscale mission in Earth's magnetosphere that challenge this paradigm. Data analysis reveals that hydrogen and helium ions, which commonly coexist, behave differently in ion-scale waves, with hydrogen ions responding more like electrons and helium ions behaving more like ions. These differential responses then induce interactions between the two ion species, exciting lower-hybrid electrostatic waves and consequently driving energy cascading from the ion scale down to the lower-hybrid scale. This process remains efficient even when helium ions are present in very minor quantities. Our observations, therefore, provide direct experimental evidence for cross-scale energy transfer processes in plasmas through multiple-ion interactions.