The equivalent current of an SC main impulse in the polar region is found to be a single vortex current system which encircles the cusp region when the interplanetary magnetic field is continuously northward in the vicinity of an interplanetary shock wave. The twin vortex current pattern hitherto believed as the current system of the main impulse tends to appear when the IMF is southward. We present evidence suggesting that this IMF dependence of the current pattern comes from the superposition of the twin vortex structure of the DP2 current or the substorm current system on the single vortex system of the main impulse.