Ion Behavior at Shocklets: A Case Study of MMS Observations

The existence of shocklets, a kind of solitary structure, has been demonstrated in the Earth's foreshock regions for decades. Their formation and evolution are believed to be controlled by ions. However, the detailed behavior of ions at them has not been well investigated observationally yet. Here, we investigate a shocklet observed by the Magnetospheric Multiscale mission in the Earth's foreshock. Analysis of ion observations reveals that the solar wind (SW) ions are bunched in gyrophase space when interacting with the whistler precursor of the shocklet, suggesting the occurrence of cyclotron resonance between them. A more detailed examination suggests that the cyclotron resonance induces a net energy flow from the whistler precursor to the SW ions. Thus, the observations presented here indicate that the cyclotron resonance between shocklet whistler precursors and the SW ions could provide a mechanism for shocklet dissipation and SW ion energization.