Latitudinal structure of the nightside region 1 field-aligned current observed from the EXOS-D satellite

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The average pattern of region 1 field-aligned current (FAC) of IIJIMA and POTEMRA (J. Geophys. Res., 81, 2165, 1976a; J. Geophys. Res., 83, 599, 1978) spans 2-3° in magnetic latitude. Using the magnetic field data acquired with the EXOS-D satellite, we notice that sharp variations of magnetic field perturbation, showing the high current densities of several micro-amperes per squared meter at the ionospheric height, are included in the region 1 current zone. In this paper, we focus our analysis on a sharp gradient of magnetic field change within 1° in latitude. In the 20-04 MLT sector, the thin current sheet as an important part of the region 1 FAC system is often found to be just equatorward of the boundary FAC system (FuKUNism et al, J. Geophys. Res., 98, 11235, 1993). A statistical survey of more than 200 satellite's crossings of the region 1 zone (in the 17-05 MLT range) shows that about 60% of the identified region 1 current systems have the latitudinal structure of a thin (on average, 0.5°) current sheet with current intensity greater than one third of the total FAC intensity of the region 1 system.
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