We present in this paper an investigation of interplanetary magnetic field (IMF) By-related plasma convection and cusp field-aligned currents using Fast Auroral Snapshot (FAST) data and the assimilative mapping of ionospheric electrodynamics (AMIE) model during a prolonged interval with large positive IMF By and northward Bz conditions (By/Bz ≫ 1). Using the FAST single trajectory observations to validate the global convection patterns at key times and key locations, we have demonstrated that the AMIE procedure provides a reasonably good description of plasma circulations in the ionosphere during this interval. Our results show that the plasma convection in the ionosphere is consistent with the antiparallel merging model. When the IMF has a strongly positive By component under northward conditions, we find that the global plasma convection forms two cells oriented nearly along the Sun-Earth line in the ionosphere. In the Northern Hemisphere, the dayside cell has clockwise convection, mainly circulating within the polar cap on open field lines. It appears to be the lobe cell convection. A second cell with counterclockwise convection is located in the nightside, circulating across the polar cap boundary. The lobe cell convection is driven by the reconnection along the antiparallel merging lines poleward of the cusp extending toward the duskside when IMF By/Bz ≫ 1. The field-aligned currents in the cusp region flow downward into the ionosphere. The return field-aligned currents extend into the polar cap in the center of the dayside lobe convection cell. The field-aligned currents are closed through the Peterson currents in the ionosphere, which flow poleward from the polar cap boundary along the electric field direction.