Plasma waves in the high-speed electron flows

Plasma waves are widely observed in the Earth's space, playing a crucial role in the heating and accelerating plasmas, cascading turbulent energy, and facilitating energy conversion during magnetic reconnection. Simulation studies have shown that high-speed electron beams can excite various plasma waves under different plasma environments. High-speed electron flows (HSEFs) are mainly observed near the X-line and drive various types of plasma instabilities and plasma waves, affecting the electron-scale dynamics. We have conducted a systematic statistical study of the super-Alfvénic high-speed electron flows in the Earth magnetotail, using NASA’s Magnetospheric Multiscale (MMS) mission observations from 2017 to 2021, and finally identified 642 events characterized by the electron bulk speed exceeding 5000 km/s. In the vicinity of the HSEFs, various types of electrostatic and electromagnetic waves are observed, their types, characteristics, and relationship with the HSEFs remain unknown. Here, we firstly perform a statistical analysis of the plasma waves in the HSEFs. Only 38.6% and 43.3% of the HSEFs are associated with parallel and perpendicular electric field waves, respectively. For the parallel electric field fluctuations, 60% of them have their frequencies between 0.1 and 1 electron cyclotron frequency (fce), which may be attributed to electrostatic solitary waves driven by electron two-stream instability. For the perpendicular electric field fluctuations, 76.6% of them have their frequencies concentrated around low hybrid frequency, possibly related to the density depletion at the speed peak of HSEFs. For both parallel and perpendicular electric field fluctuations with frequencies larger than fce, they are mainly observed near the neutral sheet, corresponding to langmuir waves and upper hybrid waves, respectively.