Localized Upper-Hybrid Waves near the Magnetic Reconnection X Line

Magnetic reconnection is a fundamental and universal process, which transfers energy stored in the magnetic field to kinetic energy of charged particles. NASA's Magnetospheric Multiscale (MMS) mission was designed to investigate the electron diffusion region (EDR), which is the core region of the reconnection where the magnetic field lines break and reconnect. Various types of plasma waves have been reported in and near EDRs. Here, we report an MMS encounter of an X line with simultaneous observations of two spacecraft locating at the neutral line and two spacecraft locating on the northern and southern sides of the EDR. MMS observe electron-scale intense current sheet, with a half-width of 1.4 electron inertial length. Large-amplitude nonlinear upper-hybrid (UH) waves are observed on both sides of the neutral line, and have highly localized distribution along the normal distance away from the neutral line. The UH waves with harmonics are produced by nonlinear interaction between the meandering electrons around the electron-scale current sheet and the inflowing counter-streaming electrons. The interaction between the electrons and the UH waves may change the electron pressure tensor and thus modify the magnetic reconnection rate.