Chorus waves, discovered as a series of repetitive coherent emissions, are well known for producing hazard radiation environments in Earth's magnetosphere. Its repetitive nature directly causes the rapid modulation of pulsating aurora in the upper atmosphere and microburst in the ionosphere, but its origin remains a decades-old puzzle. Recent simulation suggested the energetic electrons, injected from the magnetotail and drifting eastward around the Earth, are critical to form the repetitive pattern. Based on a survey of 1.5-year data from Van Allen Probes, we provide the first observational evidence for the origin of repetitive chorus emissions. We find there exists universally an inverse correlation between the repetitive period and drift velocity of energetic electrons, uncovering how the drifting energetic electrons tune the repetitive period of chorus emissions, which is also supported by kinetic simulations. Our finding may apply to understand the generation of repetitive emissions in planetary magnetospheres and laboratory plasmas.
 

chorus waves