DYNAMIC-RESPONSE OF THE CUSP MORPHOLOGY TO THE INTERPLANETARY MAGNETIC-FIELD CHANGES - AN EXAMPLE OBSERVED BY VIKING

We present a unique Viking cusp observation during a period when the i nterplanetary magnetic field (IMF) observed by IMP 8 made a steplike c hange from southward to steady northward and subsequently back to stea dy southward again. The solar wind density and velocity were fairly st eady during the entire period. Viking detected two independent plasma signatures. The first (equatorward part) is associated with southward IMF, and the second (poleward part) is associated with northward IMF. In the transition from the first signature to the second one, the ion data show an ''overlapping injection'' signature, i.e., two independen t plasma populations exist on the same field lines and are well separa ted in the energy domain at any time and at any pitch angle. The secon d plasma population is found on the higher-energy side of the first po pulation. A good relation between keV ion morphology and the field-ali gned current pattern exists even though the particle cusp is undergoin g a dynamic change. We discuss two possible scenarios for the present observation which involve the dynamic reaction of the cusp morphology to the stepwise change in the IMF condition.