Longitudinally propagating arc wave in the pre‐onset optical aurora

We present the first systematic observational evidence for a traveling periodic structure in the pre‐onset optical aurora – the longitudinally propagating arc wave (LPAW) – associated with flapping oscillations in the magnetotail. The LPAW is characterized by azimuthally moving intensity enhancements inside auroral arcs as seen by THEMIS ground‐based all‐sky imagers. It travels westward in the pre‐midnight auroral sector during the 10–20 minutes preceding auroral breakup with a velocity of 2–10 km/s, time period 40–110 s, and wavelength 250–420 km. Magnetically conjugate measurements by THEMIS satellites show low frequency plasma oscillations consistent with the parameters of the arc wave in the course of current sheet thinning. When mapped into tail, wavelength (4800–9400 km) and velocity (70–190 km/s) of the LPAW are compatible with observations and theoretical predictions for current sheet flapping motions. Our results strongly suggest that LPAW is an auroral footprint of the drift wave mode (kink, sausage, ballooning, etc.) in a stretched magnetotail.