Abstract
The distribution of field-aligned currents (FAC) during quiet and disturbed periods is calculated with the Magnetogram Inversion Technique (MIT). The polar cap boundary is determined as the high-latitude boundary of the region 1 FAC. Statistical results on the location and size of the polar cap obtained from MIT are compared with those from two independent methods: one based on Viking auroral images and the other on mapping using the Tsyganenko 1987 magnetic field model. The three methods yield essentially the same results. The northern polar cap is shifted towards dawn or dusk, depending on the sign of the By component of the interplanetary magnetic field (IMF). The polar cap area grows by a factor of 2 to 3 during substorms.
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