Using the magnetic field data from southern passes of Magsat for which the interplanetary magnetic field (IMF) Bz component was positive and the By component was particularly stable, we present a new By-controlled field-aligned current system which develops in the midnight sector. This current system is composed of three sheets of the field-aligned current approximately parallel to the auroral oval. This is referred to as the midnight sector triple-sheet (MTS) field-aligned current system. The MTS current system appears in the premidnight sector (postmidnight sector) of the southern hemisphere when By is negative (positive). When By is negative (positive), the current system consists of the most equatorward and the most poleward sheets with currents flowing away from (into) the ionosphere and a sheet in between with currents flowing into (away from) the ionosphere. The MTS current system develops at higher latitudes than the usual region 1 and region 2 field-aligned current system. When IMF Bz takes negative values or values close to zero, the region 1 and region 2 current system develops, and the MTS current system itself disappears. The MTS current system is defined in 70 passes among 100 passes for which IMF Bz was positive and By was stable. In most of the remaining 30 passes, we can see the region 1 and region 2 field-aligned current system that is similar to what is observed for southward IMF. Statistical results from the 70 cases indicate that the main controlling factors of the intensity of the MTS current system are magnitude of By and the magnetic local time. The intensity of the current system increases with ‖By‖, and the current system having a strong intensity often appears approximately at 0200 or at 2200 MLT, depending on the sign of By. ¿ American Geophysical Union 1992