Waves Generated by Electron Beam in a Crater-Shaped Flux Rope

A recent MMS observation on May 5. 2017, found a crater-shaped flux rope on the dawnside tailward magnetopause showing depression of magnetic field at the center. A previous study by Hwang et al. (2020) revealed that the flux rope is generated by a reconnection between magnetopause and magnetosphere induced by nonlinear Kelvin-Helmholtz instability. We investigated high-frequency waves inside the depression of the magnetic field. We found that there exist parallel and perpendicular electrostatic wave modes inside the crater. They can be distinguished by their locations and frequencies. The parallel mode exists at the center of the crater, and its power spectrum peaks below Fce (electron cyclotron frequency). In contrast, the perpendicular mode exists in the outer region associated with the magnetic depression, and its power spectrum peaks near Fce. The linear analysis of kinetic instability using a generalized dispersion solver shows that the parallel mode is generated by the electron beam drifting along the coaxial field line. However, the generation mechanism of the perpendicular mode is not clear yet, which requires further stud