Stormtime Magnetospheric Processes Associated With the Dawnside Current Wedge

Abstract The intensification of the westward auroral electrojet (AEJ) in the dawn sector is a characteristic feature of the storm main phase. It is considered an ionospheric segment of a wedge‐type current system, the dawnside current wedge (DCW), formed by the disruption (short‐circuiting through the ionosphere) of a magnetospheric equatorial current. The present study observationally investigates associated magnetospheric processes with a focus on near‐Earth dipolarization and tail magnetic reconnection. It is found that as the dawnside AEJ intensifies, (a) geosynchronous magnetic field dipolarizes in the dawn sector, with its region expanding eastward from the nightside, and (b) tail reconnection becomes active in the dawnside plasma sheet, with an X line retreating tailward as suggested by negative‐to‐positive transitions of V ⊥, X and B z . (a) suggests that the DCW initially develops as a substorm current wedge expanding from the nightside. (b) implies that the corresponding dipolarization region expands dawnward as earthward reconnection outflows transport magnetic flux from the plasma sheet. It is suggested that the DCW develops following an enhancement of ionospheric conductance due to the precipitation of energetic electrons, which drift eastward after being injected by a nightside substorm. As the magnetic configuration becomes more dipolar, earthward reconnection outflows may be deflected farther dawnward, which possibly sustains the dawnward expansion of the DCW. At the same time, however, the pile‐up of magnetic flux may lead to the retreat of the X line. Therefore, the DCW likely evolves as a consequence of a complex two‐way coupling between the near‐Earth current reduction and tail reconnection.