Magnetic field data observed by MAGSAT at altitude between 250--550 km clearly showed that a pair of vertical currents appears in the dayside polar cap when the interplanetary magnetic field (IMF) has a strong northward component. The currents were approximately symmetric with respect to the noon meridian, and flowed into the ionosphere on the duskside and away from it on the dawnside. These currents were very clearly seen on May 10, 1980, when the IMF was kept strongly northward all day long. The horizontal magnetic field along 10 successive orbits on the day indicated the signature. The distribution of the vertical magnetic field observed along the same orbits was reasonably explained in terms of the ionosphere Hall current which was produced when the vertical current flowed into or away from the ionosphere. The existence of such vertical currents was also confirmed by the statistical analysis over the whole MAGSAT data period.