Instantaneous, not statistical, convection patterns in the polar ionosphere are examined for periods of the northward interplanetary magnetic field (IMF) on the basis of the equivalent ionospheric current systems obtained from data of the IMS ground magnetometer. The equivalent current system is sufficient for this purpose, as long as one knows the relationship between the equivalent current system and the electrostatic potential distribution for such relatively quiet periods when no great spatial gradients in the ionospheric conductivity are expected to be present in the polar cap. In spite of the complexity of individual patterns, the following characteristic features are found. (1) During the late recovery phase of substorms, the clockwise current vortex in the postmidnight sector decays more rapidly than the counterclockwise one in the dusk sector. (2) As the IMF B_z component increases, a current system consisting of two current vortices with a sunward convection flow between them appears in the polar cap region. (3) As the IMF B_z component decreases, the clockwise current vortex in the afternoon sector decays rapidly while the counterclockwise vortex in the noon sector is shifted toward the afternoon sector. In the meantime, a new clockwise vortex emerges in the nightside and becomes enhanced.
Thus the whole current system evolves gradually into the familiar two-cell type with an antisunward convection flow over the polar cap. In particular, it is important to note that all these changes occur before the IMF turns southward. (4) The effect of the IMF B_y dependence on the equivalent current system is also confirmed on our instantaneous data base during a prolonged B_z positive condition.