Revisiting the Partial Ring Current Model:Longitudinal Asymmetry of Ground Magnetic Depression During Geomagnetic Storms

During the main phase of geomagnetic storms, the horizontal ground magnetic component is more depressed at dusk than at dawn, which was originally explained by the partial ring current model. Later studies cast question on the idea that the ring current intensifies primarily at dusk, and the cause of this dawn-dusk asymmetry still remains to be understood. The present study targets this old issue by statistically examining SuperMAG geomagnetic (sub-)indices. The results are summarized as follows: (1) the dawn-dusk asymmetry is correlated with the enhancement of the dawnside westward auroral electrojet (AEJ); (2) its correlation with the duskside eastward AEJ, which was considered by the classical partial ring current model, is insignificant; (3) the magnetic depression tends to be larger on the dayside than on the nightside, and the asymmetry is correlated with the westward AEJ in the midnight sector; (4) the dawn-dusk asymmetry enhances typically for 1~2 h, and the day-night asymmetry for < 1 h; (5) the dawn-dusk and day-night asymmetries are not correlated with each other. It is suggested that the dawn-dusk asymmetry is associated with a dawnside wedge current system with an enhanced westward AEJ (1, 2), and it is suggested to be driven by a substorm-like process, and possibly also by enhanced global convection (4). The day-night asymmetry is probably associated with the formation of the substorm wedge current system (3, 4). Although the associated dawnside and midnight westward AEJs may often enhance simultaneously, they are possibly driven by different processes (4, 5).