Ultrafast electron acceleration by space chorus

Chorus emissions, among the strongest electromagnetic waves occurring in nature, have been believed to cause intense electron acceleration which can significantly alter space weather and climate. However, direct measurement of electron acceleration by chorus has been highly challenging due to small spatiotemporal scales involved. Here we present ultrafast measurements of electron acceleration by space chorus, using high-cadence observations from NASA's Magnetospheric Multiscale spacecraft. We find that the observed chorus is associated with locally-accelerated energetic electrons exhibiting butterfly pitch angle distribution, and the chorus waves were locally damping, resulting in energy gain of electrons mainly at direction perpendicular to ambient magnetic field by cyclotron resonance. We demonstrate that the chorus-driven electron acceleration was ultrafast, with a net acceleration ratio of approximately 300 eV/s, and reveal that the electron acceleration was established in association with nonlinear wave trapping, controlled by spatiotemporal inhomogeneity of wave frequency and magnetic field. These observations provide direct evidence for nonlinear, ultrafast electron acceleration by chorus, suggest that future space weather forecasting models should incorporate chorus-induced nonlinear effects, and offer important insights into understanding energetic radiations in geospace and beyond.