Dayside four-sheet field-aligned current system during IMF B _y-dominated periods

When the dawn-dusk component of the interplanetary magnetic field (IMF B_Y) is dominant, in particular, for northward IMF, there often appears a field-aligned current (FAC) system consisting of four latitudinally separated and longitudinally elongated current sheets in the prenoon or postnoon sector, depending on the polarity of IMF By. A new explanation is proposed for the four-sheet FAC system in terms of ionospheric convection which includes an exchange cell. In this configuration, in addition to the usual round and crescent convection cells seen during IMF B_Y-dominated periods, a small convection cell called the primary exchange cell and having the same circulation direction as the round cell appears in the Northern Hemisphere prenoon (for BY < 0) or. postnoon (for By > 0) sector, equatorward of the round cell. A case study is presented in which a four-sheet FAC system is successfully explained using the exchange cell configuration. In the observations, however, it often is the case that the primary exchange cell may lose its identity because it is incorporated into the round cell to form one large, round cell. We suggest that this incorporation arises from diffusive processes in the magnetosphere-ionosphere coupling.