Observations of whistler waves in two sequential flux ropes at the magnetopause

Two ion-scale flux ropes (FRs) embedded in southward flow at the dayside magnetopause were encountered by Magnetosphere Multiscale (MMS) spacecraft sequentially in 9 s. Super-Alfvenic electron jet, non-zero electric field in electron frame, and strong current are observed in these two FRs. Specifically, whistler waves are observed inside and outside the FRs. The properties of whistler waves inside the FRs are quite different from whistler waves outside the FRs. Whistler waves outside the FRs have a frequency range between 0.1 f(ce) and f(ce) (f(ce) is the electron gyro-frequency) and they propagate parallel to the magnetic field. Whistler waves inside the FRs have a frequency range between 0.5 f(ce) to f(ce) and they propagate antiparallel to the magnetic field. The intensity of whistler waves outside FRs is stronger than the whistler waves inside FRs. Electron beam instability may be the reason for the generation of whistler waves observed here. The difference between whistlers inside and outside FRs may have a close relationship with different magnetic topology and electron distribution inside and outside FRs.