MMS Observations of Broadband Electrostatic Waves in Electron Diffusion Region of Magnetotail Reconnection

In this study, the broadband electrostatic waves are observed in electron diffusion region (EDR). Detailed analysis has shown that the parallel electric field of these waves have a bipolar or continuous steepening waveform. The bipolar structures of the parallel electric field are usually accompanied by electron holes (EHs) in phase space. The parallel spatial scales l|| of EHs are similar to 3-5 lambda D (lambda D is the Debye length), and the perpendicular spatial scales l perpendicular to are larger than similar to 18.6 lambda D. Therefore, the length ratios l perpendicular to/l|| are larger than similar to 3.7-6.2, which means that the EHs have oblate shapes. Associated with these plasma waves, electron beams propagating away from the X-line have been observed. By performing a one-dimensional (1-D) electrostatic particle-in-cell simulation of the bump-on-tail instability, we reproduce two types of waveforms similar to those observed in the EDR during the different evolution stage of the instability, which implies that these electrostatic waves are generated by the bump-on-tail instabilities.