We present the direct connection between the wave-like auroral structure around the time of auroral expansion onset and the ballooning mode waves in the near-Earth magnetotail. Based on the NASA mission time history of events and macroscale interactions during substorms (THEMIS) ground-based all-sky imagers, we show that around the time of auroral expansion onset, a wave-like auroral structure first has four luminosity peaks separated by 2-3° magnetic longitude (MLON). Subsequently, the wave-like structure propagates in the azimuthal direction and an overall bright arc spans approximately 1 h magnetic local time. The wavelength is estimated to be 120-180 km. Finally, a noticeable poleward auroral expansion is observed. The ballooning mode waves are identified by two THEMIS probes in the near-Earth magnetotail. The observed wavelength of the ballooning mode waves is approximately equal to the order of the ion Larmor radius. The wavelength of 1500 3000 km in the near-Earth magnetotail is comparable with the wave-like auroral structure estimate. This study suggests that the ballooning mode waves might play a crucial role in auroral expansion onset, corresponding to the wave-like auroral structure in this study.%We present the direct connection between the wave-like auroral structure around the time of auroral expansion onset and the ballooning mode waves in the near-Earth magnetotail.Based on the NASA mission time history of events and macroscale interactions during substorms (THEMIS) ground-based all-sky imagers,we show that around the tirne of auroral expansion onset,a wave-like auroral structure first has four luminosity peaks separated by 2-3° magnetic longitude (MLON).Subsequently,the wave-like structure propagates in the azimuthal direction and an overall bright arc spans approximately 1 h magnetic local time.The wavelength is estimated to be 120-180 km.Finally,a noticeable poleward auroral expansion is observed.The ballooning mode waves are identified by two THEMIS probes in the near-Earth magnetotail.The observed wavelength of the ballooning mode waves is approximately equal to the order of the ion Larmor radius.The wavelength of 1500 3000 km in the near-Earth magnetotail is comparable with the wave-like auroral structure estimate.This study suggests that the ballooning mode waves might play a crucial role in auroral expansion onset,corresponding to the wave-like auroral structure in this study.Recent theoretical models predict that ballooning instability is a possible trigger of the substorm expansion phase.[1] The idea that ballooning instability in the near-Earth tail is a substorm trigger was first proposed by Roux et al.[2] Since then,many theoretical and observational studies have been conducted to see whether or not ballooning instability plays a crucial role in the near-Earth tail during the late growth phase of substorms.[3]
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