Substorm currents associated with magnetotail magnetic dipolarization: Geotail observations

Three typical events of magnetic field dipolarization in the near-Earth mag netotail observed by the Geotail satellite are critically studied. During t hese events, Geotail was in the region R = 10-18 R-E in the midnight sector . Transient, not step-like negative, perturbations in the B-y component wer e observed at Geotail just after the boundary crossings from the southern t ail lobe to the plasma sheet, suggesting the existence of double sheet curr ents: a pair of earthward and tailward field-aligned sheet currents with th e earthward current on the poleward side. Geotail simultaneously observed a n intense northward electric field, which indicates that a dynamo region fo r the double sheet current exists tailward of the spacecraft. Examining mag netograms at ground stations, it is found that the ionospheric foot points of Geotail were collocated with the region of the enhanced westward electro jet. On the basis of these observations, we propose a new substorm current model in which a double sheet current system is coupled with the convention al substorm wedge current system.