Ultrafast observations of electron dynamics during falling-tone chorus emissions in space

Chorus waves are electromagnetic emissions widely occurring in planetary (e.g., Earth, Mars, and Jupiter) environments and can cause hazardous space weather effects which threaten human safety and man-made electromagnetic devices. For decades, chorus waves have been intensively investigated, both experimentally and theoretically, in the context of planetary dipolar fields. However, the underlying electron dynamics which governs the wave generation and evolution, has not been well diagnosed by in situ observations, due to the limited capacity of previous spacecraft missions in geospace. Here we report unexpected observations of repetitive, falling-tone chorus waves in the terrestrial neutral sheet. Using unprecedently high-resolution data from NASA’s MMS mission, we present ultrafast measurements of electron dynamics and electric current within the waves, finding energy transfer from local electrons to the waves and development of electron hill in the wave phase space. The results provide smoking-gun evidence for the nonlinear generation mechanism for falling-tone chorus waves, and reveal that magnetic field inhomogeneity, which has been traditionally believed to be crucial for chorus generation, is not necessarily required. The current observations provide important new insights into understanding energetic electron dynamics in space.