On the origin of the cusp field-aligned currents
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A complicated system of field-aligned currents is known to exist in the high-latitude region near noon, associated with the cusp. We suggest that the equatorward part of this system, referred to as the Region 1 field-aligned currents, is caused by the leakage of the field-aligned currents associated with the rotational discontinuities at the dayside magnetopause. The poleward part, referred to as the cusp field-aligned currents, is associated with the tail magnetopause. In this situation, it can be shown that the direction of the field-aligned currents at the magnteopause is controlled by the y-component of the interplanetary magnetic field (By). The poleward (equatorward) part of this field-aligned current system is found to flow out of (into) the northern polar ionosphere when By > 0 and into (out of) the northern ionsophere when By < 0. This current pattern reverses systematically in the southern polar ionosphere. Therefore, the suggested mechanism can explain qualitatively the observed changes of the cusp current systems. Further, the latitudinal width of the cusp field-aligned current system at the ionospheric altitude is estimated to be 100-400 km, consistent with observations.
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