Periodic black auroral patches at the dawnside dipolarization front during a substorm

Periodic black auroral patches were observed in the Canadian postmidnight sector by ground‐based all‐sky cameras during an auroral substorm. The aurora consisted of several black patches of ∼50 km (∼1° in longitude) scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from Gillam to Sanikiluaq ground stations 15 min after the auroral intensification occurred in Alaska. To the best of our knowledge, this type of black auroras has never been reported before, although the surrounding luminous auroras evolve into the well‐known torch structures later. In situ observations by the THEMIS satellites, three of which were close to being magnetically conjugate in the plasma sheet, as well as ground‐based proton auroral observations, show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance. The flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions. We speculate, based on manifestations of high‐pressure regions in dipolarized flux tubes, the pressure‐driven interchange instability formed a beading shape (periodic black patches) of field‐aligned currents. The black auroral patches developed into torch structures after the flow disappearance, indicating the dawnward flows might stabilize and limit the instability within a narrow L shell where the pressure gradient is directed Earthward and flows cannot enter. We conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm.