Equatorward and poleward expansion of the auroras during auroral substorms

Using all‐sky TV auroral data with high spatial and temporal resolution, the formation of the auroral bulge is investigated. We show how the discrete auroral structures within the poleward expanding bulge develop systematically toward the west, the east, and also equatorward from a localized breakup region. Auroral structure at the western end of the bulge (a surge) develops with clockwise rotation as viewed along the magnetic field direction. At the eastern part of the bulge, thin auroral features propagate eastward from the breakup region. Around the central meridian of the bulge, auroral features expand equatorward and become north‐south aligned (the N‐S aurora). The N‐S aurora and the eastward propagating aurora develop into diffuse and pulsating aurora after the expansion. We suggest that these discrete auroral structures in the bulge develop along the plasma streamlines in a localized distorted two‐cell equipotential distribution; the negative potential peak is concentrated at the western part of the bulge and the positive potential region is spread poleward of the eastward propagating aurora. Equatorward expansion of auroral structures would be attributed in this scenario to the subsequent earthward transport of plasma from the onset region. Energetic particles would also be injected at the N‐S auroral region as well as to the east of the region and would trigger the pulsating auroral activity. Poleward expansion, on the other hand, would be produced by the tailward motion of the onset region in accordance with the tailward moving current disruption region.