Polar cap currents for different directions of the interplanetary magnetic field in the Y-Z plane

Abstract

During intervals of a steady state condition of the interplanetary magnetic field the geomagnetic variations at the polar cap have been examined as a function of the direction of the interplanetary field in the Y-Z plane of the geocentric solar magnetospheric coordinate system. A prominent feature of the equivalent current patterns during a positive B/sub z/ steady state is the total vanishing of the well-known two-cell current system, which is generally considered to be related to the convection across the polar cap. Another significant effect for a northward interplanetary magnetic field is a poleward shift of the B/sub y/ associated currents at the day side of the polar cap, with no decrease of the current strength. The existence of these currents even during the absence of the two-cell current system indicates that the explanation proposed by Heppner (1972) of the currents' being due to a shift toward the dawn or the dusk of the polar cap convection is not exhaustive. Rather, the magnetic perturbations for a positive B/sub z/ may correspond to merging of the northward interplanetary magnetic field with the open geomagnetic field lines at the poleward boundary of the cusp as suggested by Burch (1973). A pronounced decreasemore » of the B/sub y/ related magnetic perturbations from summer to winter compared to the seasonal variation of the B/sub z/ related disturbances at the polar cap indicates a different origin of the associated currents. (auth)« less