Subion-Scale Flux Rope Nested Inside Ion-Scale Flux Rope in Earth's Magnetotail

Magnetic flux ropes (MFRs)-transient magnetic structures widely observed in Earth's magnetosphere-play crucial roles in particle acceleration and energy dissipation. MFRs above ion scale have been well studied, while MFRs at subion scales and their coupling with larger-scale ropes still remains not well understood. In this paper, we present the first observation of a subion-scale MFR nested in an ion-scale MFR, using high-resolution data from the magnetospheric multiscale (MMS) mission. We find that the subion-scale MFR hosts more intense plasma activity than the ion-scale MFR, including distinct electron agyrotropy and strong electromagnetic turbulence. In addition, axis of the subion-scale MFR is oblique to that of the ion-scale MFR, indicating that their generation may be ascribed to 3D magnetic reconnection. Observations of such cross-scale structure may shed new lights into understanding energy cascade in space plasmas.