The relationship between ionospheric and field‐aligned currents in the dayside cusp

Observations of field‐aligned currents determined from magnetic disturbances measured with the Triad satellite near local magnetic noon have been compared to simultaneous magnetic perturbations observed at the ground with a dense chain of magnetic observatories located along the west coast of Greenland. The investigation reveals that the equivalent ionospheric currents at the dayside polar cusp are located between two field‐aligned current sheets and the direction of the equivalent ionospheric current depends upon the directions of the two field‐aligned currents in a manner consistent with the assumption that the equivalent ionospheric current represents an ionospheric Hall current. Previous investigations have shown that the direction of the ionospheric cusp current is determined by the azimuthal component By of the interplanetary magnetic field (IMF). A similar relation therefore exists between the field‐aligned currents and By in the dayside cusp region. When By is positive, the ionospheric current (the DPY current) flows eastward, and the flow of the poleward field‐aligned current (the cusp region field‐aligned current) is away from the ionosphere in the northern hemisphere. When the polarity of By reverses, the flow directions of the ionospheric and field‐aligned currents reverse.