Flapping motion of onset aurora

Abstract. Onset aurora is meso-scale auroral forms (100s km) of spiral aurora arising out of the equatorward arc in association with the initial pulse of Pi2 pulsation, i.e., onset of field line dipolarization. Following the auroral onset, the head of the equatorward arc rapidly turns in a clockwise direction with expansion poleward, and during the second event of onset aurora, the whole of the arc rotates clockwise as viewed along the field lines. We model the deformation of onset aurora as an ExB drift of negatively charged solitary potential area (ion hole) in the polar ionosphere. It is suggested that twist motion of the onset aurora is analogous to the flapping motion of transmission belt in a factory driven by rotating line shaft. Like fluctuations in the line-of-sight velocity of the belt, non-uniform plasma flows in the ion hole trigger flapping motion of the arc.