Mapping of the ionospheric field-aligned currents to the equatorial magnetosphere

The assimilative mapping of ionospheric electrodynamics (AMIE)-derived ionospheric field-aligned current patterns during the ninth Coordinated Data Analysis Workshop substorm event have been mapped to the magnetospheric equatorial plane, using the data-based magnetospheric magnetic field model [Tsyganenko, 1995]. We find that at the onset of the substorm expansion phase, a pair of field-aligned currents emerges about 8 R_E tailward of the Earth, near the x axis. This pair of field-aligned currents is consistent with the substorm current wedge model; it lines up in the east-west direction across the near-Earth tail current and has the sense of the nightside region 1 current, i.e., flowing into the ionosphere on the east side and out of the ionosphere on the west side. The ionospheric counterpart of the substorm wedge currents in the midnight sector, on the other hand, has a northeast-to-southwest alignment. During the expansion phase, the substorm wedge currents intensify and expand in spatial size; however, their peaks remain nearly at the same location in the equatorial magnetosphere, about 8 R_E tailward from the Earth. We also find that the intensification of the substorm wedge currents during the expansion phase is accompanied by the intensification of the region 2-sense currents located closer to the Earth, and the boundary between the region 2 and substorm wedge currents lies around x = -6 R_E.