East‐west type precursor activity prior to the auroral onset: Ground‐based and THEMIS observations

Using ground-based optical observations, we study an auroral breakup event, focusing on the wave-like signatures of the east-west (E-W) type auroral activities which appear before breakup. By conjunction with the Time History of Events and Macroscale Interactions during Substorms (THEMIS) P2 and P5 measurements, it is shown that the underlying wave mode can be identified as the ballooning mode. Considering the similarity of the wave-like characteristics derived from ground-based auroral and THEMIS spacecraft observations, we argue that the E-W activities under study may be related to ballooning waves propagating in the plasma sheet. The implications for mechanisms of substorm triggering are discussed.