Periodic aurora surge propagating eastward/westward at poleward boundary of aurora zone during the first 10Â min intervals of Pi2 onset

► Surge propagated at poleward boundary of aurora zone. ► Surge is an auroral manifestation of the bi-directional flow at the outer boundary of the inner magnetosphere. ► Bi-directional flow at the outer boundary is the “external stimulus” for the cavity oscillations.