Magnetotail Source of Premidnight Upward Field‐Aligned Currents: Nature of the Substorm Current System

The substorm current wedge constitutes the backbone of the substorm current system. Its field‐aligned current components include an upward current in the premidnight sector and a downward current in the midnight‐morning sector, which connect the auroral ionosphere to the magnetotail. Because the fundamental ionospheric manifestations of magnetospheric substorms are dynamic auroral activity and the westward electrojet, the properties of these field‐aligned currents provide key insights into magnetotail dynamics during substorms. We investigate the structure of field‐aligned currents in the near‐Earth magnetotail at radial distances of 8–15 R E using Geotail observations during the formation of the substorm current wedge. The premidnight field‐aligned currents are located in the outermost plasma sheet and form a double‐sheet structure. In the double‐sheet structure, the outer earthward currents are carried by tailward‐flowing Hall electrons, whereas the inner tailward currents are carried by earthward‐flowing accelerated electrons. These electrons are accompanied by earthward‐flowing accelerated ions in the inner tailward current layer. The observed characteristics of the current sheet structure strongly suggest that the upward field‐aligned currents are directly driven by magnetic reconnection occurring in the near‐Earth magnetotail at radial distances of 20–30 R E . The midnight–morning downward field‐aligned currents are located in the outer plasma sheet and have a thick single‐sheet structure. Although these results are largely based on Geotail observations with limited observational capabilities from the current perspective, the long‐term magnetotail survey performed by Geotail provided a sufficient number of clear‐cut events to elucidate the underlying physical processes.