Substorm currents associated with magnetotail magnetic dipolarization: Geotail observations

摘要

Three typical events of magnetic field dipolarization in the near-Earth magnetotail observed by the Geotail satellite are critically studied. During these 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 were observed at Geotail just after the boundary crossings from the southern tail lobe to the plasma sheet, suggesting the existence of double sheet currents: a pair of earthward and tailward field-aligned sheet currents with the earthward current on the poleward side. Geotail simultaneously observed an intense northward electric field, which indicates that a dynamo region for the double sheet current exists tailward of the spacecraft. Examining magnetograms at ground stations, it is found that the ionospheric foot points of Geotail were collocated with the region of the enhanced westward electrojet. On the basis of these observations, we propose a new substorm current model in which a double sheet current system is coupled with the conventional substorm wedge current system. [References: 30]